chore: migrate java8 to java21 mail

[Alan5142]

Description

Checklist

• [ ] 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

[snippet-bot[bot]]

Here is the summary of changes.

You are about to add 10 region tags.

• appengine-java21/ee8/mail/src/main/java/com/example/appengine/mail/BounceHandlerServlet.java:19, tag gae_java21_mail_bounce_handler
• appengine-java21/ee8/mail/src/main/java/com/example/appengine/mail/HandleDiscussionEmail.java:19, tag gae_java21_mail_discussion_email
• appengine-java21/ee8/mail/src/main/java/com/example/appengine/mail/MailHandlerBase.java:19, tag gae_java21_mail_handler_base
• appengine-java21/ee8/mail/src/main/java/com/example/appengine/mail/MailHandlerServlet.java:19, tag gae_java21_mail_handler_servlet
• appengine-java21/ee8/mail/src/main/java/com/example/appengine/mail/MailServlet.java:54, tag gae_java21_mail_simple
• appengine-java21/ee8/mail/src/main/java/com/example/appengine/mail/MailServlet.java:90, tag gae_java21_mail_multipart
• appengine-java21/ee8/mail/src/main/webapp/WEB-INF/appengine-web.xml:23, tag gae_java21_mail_handle_incoming
• appengine-java21/ee8/mail/src/main/webapp/WEB-INF/web.xml:31, tag gae_java21_mail_filter_handler
• appengine-java21/ee8/mail/src/main/webapp/WEB-INF/web.xml:44, tag gae_java21_mail_incoming_mail_servlet
• appengine-java21/ee8/mail/src/main/webapp/WEB-INF/web.xml:64, tag gae_java21_mail_bounced_mail_servlet

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