chore: upgrade datastore sample from java8 to java21

[Alan5142]

Description

Checklist

• [x] I have followed Sample Format Guide
• [ ] pom.xml parent set to latest shared-configuration
• [ ] Appropriate changes to README are included in PR
• [ ] These samples need a new API enabled in testing projects to pass (let us know which ones)
• [ ] These samples need a new/updated env vars in testing projects set to pass (let us know which ones)
• [x] Tests pass: mvn clean verify required
• [x] Lint passes: mvn -P lint checkstyle:check required
• [ ] Static Analysis: mvn -P lint clean compile pmd:cpd-check spotbugs:check advisory only
• [ ] This sample adds a new sample directory, and I updated the CODEOWNERS file with the codeowners for this sample
• [ ] This sample adds a new Product API, and I updated the Blunderbuss issue/PR auto-assigner with the codeowners for this sample
• [x] Please merge this PR for me once it is approved

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[snippet-bot[bot]]

Here is the summary of changes.

You are about to add 63 region tags.

• appengine-java21/ee8/datastore/src/main/java/com/example/appengine/ListPeopleServlet.java:19, tag gae_java21_datastore_cursors
• appengine-java21/ee8/datastore/src/main/java/com/example/appengine/StatsServlet.java:35, tag gae_java21_datastore_stat_example
• appengine-java21/ee8/datastore/src/test/java/com/example/appengine/EntitiesTest.java:71, tag gae_java21_datastore_kind_example
• appengine-java21/ee8/datastore/src/test/java/com/example/appengine/EntitiesTest.java:97, tag gae_java21_datastore_identifiers_1
• appengine-java21/ee8/datastore/src/test/java/com/example/appengine/EntitiesTest.java:110, tag gae_java21_datastore_identifiers_2
• appengine-java21/ee8/datastore/src/test/java/com/example/appengine/EntitiesTest.java:120, tag gae_java21_datastore_parent_1
• appengine-java21/ee8/datastore/src/test/java/com/example/appengine/EntitiesTest.java:136, tag gae_java21_datastore_parent_2
• appengine-java21/ee8/datastore/src/test/java/com/example/appengine/EntitiesTest.java:146, tag gae_java21_datastore_working_with_entities
• appengine-java21/ee8/datastore/src/test/java/com/example/appengine/EntitiesTest.java:154, tag gae_java21_datastore_creating_an_entity_1
• appengine-java21/ee8/datastore/src/test/java/com/example/appengine/EntitiesTest.java:165, tag gae_java21_datastore_creating_an_entity_2
• appengine-java21/ee8/datastore/src/test/java/com/example/appengine/EntitiesTest.java:177, tag gae_java21_datastore_retrieving_an_entity
• appengine-java21/ee8/datastore/src/test/java/com/example/appengine/EntitiesTest.java:193, tag gae_java21_datastore_deleting_an_entity
• appengine-java21/ee8/datastore/src/test/java/com/example/appengine/EntitiesTest.java:210, tag gae_java21_datastore_repeated_properties
• appengine-java21/ee8/datastore/src/test/java/com/example/appengine/EntitiesTest.java:232, tag gae_java21_datastore_embedded_entities_1
• appengine-java21/ee8/datastore/src/test/java/com/example/appengine/EntitiesTest.java:254, tag gae_java21_datastore_embedded_entities_2
• appengine-java21/ee8/datastore/src/test/java/com/example/appengine/EntitiesTest.java:277, tag gae_java21_datastore_embedded_entities_3
• appengine-java21/ee8/datastore/src/test/java/com/example/appengine/EntitiesTest.java:296, tag gae_java21_datastore_gae_batch_operations
• appengine-java21/ee8/datastore/src/test/java/com/example/appengine/EntitiesTest.java:325, tag gae_java21_datastore_generating_keys_1
• appengine-java21/ee8/datastore/src/test/java/com/example/appengine/EntitiesTest.java:341, tag gae_java21_datastore_generating_keys_2
• appengine-java21/ee8/datastore/src/test/java/com/example/appengine/EntitiesTest.java:360, tag gae_java21_datastore_generating_keys_3
• appengine-java21/ee8/datastore/src/test/java/com/example/appengine/IndexesTest.java:69, tag gae_java21_datastore_unindexed_properties_1
• appengine-java21/ee8/datastore/src/test/java/com/example/appengine/MetadataEntityGroupTest.java:72, tag gae_java21_datastore_entity_group_1
• appengine-java21/ee8/datastore/src/test/java/com/example/appengine/MetadataEntityGroupTest.java:114, tag gae_java21_datastore_entity_group_2
• appengine-java21/ee8/datastore/src/test/java/com/example/appengine/MetadataKindsTest.java:72, tag gae_java21_datastore_kind_query
• appengine-java21/ee8/datastore/src/test/java/com/example/appengine/MetadataNamespacesTest.java:73, tag gae_java21_datastore_queries_intro
• appengine-java21/ee8/datastore/src/test/java/com/example/appengine/MetadataNamespacesTest.java:102, tag gae_java21_datastore_namespace_query
• appengine-java21/ee8/datastore/src/test/java/com/example/appengine/MetadataPropertiesTest.java:74, tag gae_java21_datastore_property_query_example
• appengine-java21/ee8/datastore/src/test/java/com/example/appengine/MetadataPropertiesTest.java:110, tag gae_java21_datastore_property_filtering_example
• appengine-java21/ee8/datastore/src/test/java/com/example/appengine/MetadataPropertiesTest.java:162, tag gae_java21_datastore_property_ancestor_query_example
• appengine-java21/ee8/datastore/src/test/java/com/example/appengine/MetadataPropertiesTest.java:202, tag gae_java21_datastore_property_representation_query_example
• appengine-java21/ee8/datastore/src/test/java/com/example/appengine/ProjectionTest.java:69, tag gae_java21_datastore_grouping
• appengine-java21/ee8/datastore/src/test/java/com/example/appengine/QueriesTest.java:89, tag gae_java21_datastore_datastore_property_filter
• appengine-java21/ee8/datastore/src/test/java/com/example/appengine/QueriesTest.java:115, tag gae_java21_datastore_datastore_key_filter
• appengine-java21/ee8/datastore/src/test/java/com/example/appengine/QueriesTest.java:148, tag gae_java21_datastore_kindless_query
• appengine-java21/ee8/datastore/src/test/java/com/example/appengine/QueriesTest.java:176, tag gae_java21_datastore_ancestor_filter
• appengine-java21/ee8/datastore/src/test/java/com/example/appengine/QueriesTest.java:188, tag gae_java21_datastore_ancestor_query
• appengine-java21/ee8/datastore/src/test/java/com/example/appengine/QueriesTest.java:241, tag gae_java21_datastore_kindless_ancestor_key_query
• appengine-java21/ee8/datastore/src/test/java/com/example/appengine/QueriesTest.java:256, tag gae_java21_datastore_kindless_ancestor_query
• appengine-java21/ee8/datastore/src/test/java/com/example/appengine/QueriesTest.java:298, tag gae_java21_datastore_keys_only
• appengine-java21/ee8/datastore/src/test/java/com/example/appengine/QueriesTest.java:322, tag gae_java21_datastore_sort_order
• appengine-java21/ee8/datastore/src/test/java/com/example/appengine/QueriesTest.java:363, tag gae_java21_datastore_multiple_sort_orders
• appengine-java21/ee8/datastore/src/test/java/com/example/appengine/QueriesTest.java:399, tag gae_java21_datastore_interface_1
• appengine-java21/ee8/datastore/src/test/java/com/example/appengine/QueriesTest.java:450, tag gae_java21_datastore_interface_3
• appengine-java21/ee8/datastore/src/test/java/com/example/appengine/QueriesTest.java:480, tag gae_java21_datastore_inequality_filters_one_property_valid_1
• appengine-java21/ee8/datastore/src/test/java/com/example/appengine/QueriesTest.java:503, tag gae_java21_datastore_inequality_filters_one_property_invalid
• appengine-java21/ee8/datastore/src/test/java/com/example/appengine/QueriesTest.java:550, tag gae_java21_datastore_inequality_filters_one_property_valid_2
• appengine-java21/ee8/datastore/src/test/java/com/example/appengine/QueriesTest.java:610, tag gae_java21_datastore_inequality_filters_sort_orders_invalid_1
• appengine-java21/ee8/datastore/src/test/java/com/example/appengine/QueriesTest.java:630, tag gae_java21_datastore_inequality_filters_sort_orders_invalid_2
• appengine-java21/ee8/datastore/src/test/java/com/example/appengine/QueriesTest.java:656, tag gae_java21_datastore_surprising_behavior_1
• appengine-java21/ee8/datastore/src/test/java/com/example/appengine/QueriesTest.java:689, tag gae_java21_datastore_surprising_behavior_2
• appengine-java21/ee8/datastore/src/test/java/com/example/appengine/QueriesTest.java:722, tag gae_java21_datastore_surprising_behavior_3
• appengine-java21/ee8/datastore/src/test/java/com/example/appengine/QueriesTest.java:744, tag gae_java21_datastore_surprising_behavior_4
• appengine-java21/ee8/datastore/src/test/java/com/example/appengine/QueriesTest.java:767, tag gae_java21_datastore_single_retrieval
• appengine-java21/ee8/datastore/src/test/java/com/example/appengine/QueriesTest.java:809, tag gae_java21_datastore_query_limit
• appengine-java21/ee8/datastore/src/test/java/com/example/appengine/ReadPolicyTest.java:65, tag gae_java21_datastore_data_consistency
• appengine-java21/ee8/datastore/src/test/java/com/example/appengine/TransactionsTest.java:81, tag gae_java21_datastore_using_transactions
• appengine-java21/ee8/datastore/src/test/java/com/example/appengine/TransactionsTest.java:103, tag gae_java21_datastore_entity_groups
• appengine-java21/ee8/datastore/src/test/java/com/example/appengine/TransactionsTest.java:148, tag gae_java21_datastore_creating_an_entity_in_a_specific_entity_group
• appengine-java21/ee8/datastore/src/test/java/com/example/appengine/TransactionsTest.java:171, tag gae_java21_datastore_cross_group_transactions
• appengine-java21/ee8/datastore/src/test/java/com/example/appengine/TransactionsTest.java:195, tag gae_java21_datastore_uses_for_transactions_1
• appengine-java21/ee8/datastore/src/test/java/com/example/appengine/TransactionsTest.java:229, tag gae_java21_datastore_uses_for_transactions_2
• appengine-java21/ee8/datastore/src/test/java/com/example/appengine/TransactionsTest.java:271, tag gae_java21_datastore_uses_for_transactions_3
• appengine-java21/ee8/datastore/src/test/java/com/example/appengine/TransactionsTest.java:296, tag gae_java21_datastore_transactional_task_enqueuing

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[iennae]

/gemini review