Update Java Versions to 21 on CI (#165210)

Updated java versions to 21 on CI. Monitor to see if anything breaks.

Fixes https://github.com/flutter/flutter/issues/163158

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we expect every widget to implement].
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- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel
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---------

Co-authored-by: Gray Mackall <34871572+gmackall@users.noreply.github.com>

dev/devicelab/bin/tasks/build_android_host_app_with_module_aar.dart

@@ -457,7 +457,7 @@ Future<void> main() async {
     combine(<TaskFunction>[
       // 3 tests comes close to timeout.
       // Pre AGP 8.3
-      ModuleTest(gradleVersion: '8.4', agpVersion: Version.parse('8.1.0')).call,
+      ModuleTest(gradleVersion: '8.4', agpVersion: Version.parse('8.2.1')).call,
       // Post AGP 8.3 + rc candidates can work
       ModuleTest(gradleVersion: '8.13-rc-1', agpVersion: Version.parse('8.8.1')).call,
     ]),