[hotfix] Use correct FlinkVersion.v2_1

[snuyanzin]

What is the purpose of the change

The PR is to fix the build after
ed9aebe

Verifying this change

existing tests

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)

[gustavodemorais]

Thanks for the fix, @snuyanzin

[flinkbot]

CI report:

• 8527afc Azure: SUCCESS

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