[FLINK-33457][tests] FlinkImageBuilder checks for Java 21

[snuyanzin]

What is the purpose of the change

The PR allows to pass FlinkImageBuilder to proceed with e2e tests

Verifying this change

This change is a trivial rework

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): ( no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (no)
• The serializers: (no )
• The runtime per-record code paths (performance sensitive): (no )
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: (no )
• The S3 file system connector: (no )

Documentation

• Does this pull request introduce a new feature? (no)
• If yes, how is the feature documented? (not applicable)

[flinkbot]

CI report:

• 8acb884 Azure: SUCCESS

Bot commands

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• @flinkbot run azure re-run the last Azure build

[1996fanrui]

Thanks @snuyanzin for the effort!

LGTM

[snuyanzin]

Thanks for taking a look