[hotfix][annotations] Fix the Flink version enum for v1.18

[LadyForest]

What is the purpose of the change

This is a trivial fix for the FlinkVersion enum, which should start with lowercase.

Brief change log

Change V1_18 to v1_18.

Verifying this change

This change is a trivial fix without any test coverage.

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:

• 156e8ec Azure: SUCCESS

Bot commands

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

[leonardBang]

+1, lest's wait the CI green