[FLINK-39829][build] Bump scala to 2.12.21

[snuyanzin]

What is the purpose of the change

Bumping scala to 2.12.21
as based on https://docs.scala-lang.org/overviews/jdk-compatibility/overview.html
it is better to have 2.12.21 for the case of java 25
also it is binary compatible (as mentioned at https://github.com/scala/scala/releases/tag/v2.12.21)

Brief change log

pom, NOTICE

Verifying this change

existing tests

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

• Dependencies (does it add or upgrade a dependency): (yes )
• 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)

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Was generative AI tooling used to co-author this PR?

• Yes (please specify the tool below)

[flinkbot]

CI report:

• 45c1546 Azure: SUCCESS

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[MartijnVisser]

I did a check on the Scala upgrade and I think this is safe to merge, because:

1. The Scala release notes state ""Scala 2.12.21 is binary-compatible with the whole Scala 2.12 series".
2. The reason why Flink was pinned to 2.12.7 for such a long time was because 2.12.8 broke closure/lambda serialization, and that surface lived in the Scala DataStream/DataSet API in module flink-scala. We don't have that anymore since Flink 2.0. The Scala usage we have are flink-table-planner internals and the table-api-scala part, which has no user-facing savepoint/closure serializers.

So LGTM, assuming CI passes of course

[raminqaf]

Changes LGTM. The CI is on failure but the test seems to be a flaky one.
https://dev.azure.com/apache-flink/apache-flink/_build/results?buildId=75590&view=ms.vss-test-web.build-test-results-tab&runId=4219439&resultId=214357&paneView=debug

[snuyanzin]

now it is green, merging
thanks for taking a look