[FLINK-34149][Runtime/Checkpointing] Fix SplitEnumeratorContext compatibility issue

[yunfengzhou-hub]

What is the purpose of the change

This PR adds a default implementation to SplitEnumeratorContext#setIsProcessingBacklog to fix the compatibility issue introduced in #22931.

Brief change log

• Adds a default implementation to SplitEnumeratorContext#setIsProcessingBacklog.

Verifying this change

It has been verified that the flink-connector-kafka repo will not throw compatibility exception about this method when building against flink-1.19-snapshot after the changes in this PR are introduced. The build would still fail due to other API-incompatible changes, though, and those changes are not related to #22931.

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

[yunfengzhou-hub]

Hi @lindong28, could you please take a look at this PR?

[flinkbot]

CI report:

• a9ab0ea Azure: SUCCESS

Bot commands

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

[lindong28]

Thanks for the PR! LGTM. Will merge PR after the CI has passed.