Update gradle toolchain to Java 21

[xiang17]

Fix # .

For significant contributions please make sure you have completed the following items:

• Design discussion issue #
• Changes in public surface reviewed
• CHANGELOG.md updated

[xiang17]

Blocked on a warning from Java 8:

Task :agent:agent-gc-monitor:gc-monitor-api:compileJava
1 error
3 warnings
warning: [options] source value 8 is obsolete and will be removed in a future release
warning: [options] target value 8 is obsolete and will be removed in a future release
warning: [options] To suppress warnings about obsolete options, use -Xlint:-options.
error: warnings found and -Werror specified