chore(bigtable): update bigtable example 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 3 region tags.

• appengine-java21/ee8/bigtable/build.gradle:14, tag gae_java21_bigtable_gradle_file
• appengine-java21/ee8/bigtable/build.gradle:83, tag gae_java21_bigtable_gradle_model
• appengine-java21/ee8/bigtable/src/main/java/com/example/bigtable/BigtableHelloWorld.java:20, tag gae_java21_bigtable_helloworld

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