Bumping the engine to use Java 21.

Some Java 21 changes that affect our tests:

• Starting JDK 19, we can no longer use Mockito's mock to fake the ByteBuffer class (mockito issue here). I replaced the usage with a captor instead.
• Included in the Java 21 Release here are unicode updates that specifically affect chinese characters. It likely affects this unicode accessibility test itCanPredictCursorMovementsWithGranularityWordUnicode in AccessiblityBridgeTest.java, which uses chinese characters.

To reproduce the error in AccessiblityBridgeTest.java locally, follow these steps:

1. Run et build -c android_debug_unopt
2. Run gclient sync -D from root
3. Navigate to engine/src/flutter/third_party/java/openjdk/openjdk.cipd.yaml to make sure JDK 21 has been properly generated. If you see "OpenJDK 21," it has been properly generated.
4. Navigate to engine/src/flutter and run testing/run_tests.py --android-variant=android_debug_unopt --type=java --java-filter=io.flutter.view.AccessibilityBridgeTest

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• I followed the breaking change policy and added Data Driven Fixes where supported.
• All existing and new tests are passing.

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