Revert "[FLINK-28046][connectors] Mark SourceFunction interface as @deprecated"

[ruanhang1993]

What is the purpose of the change

This PR reverts commit 07bf511, which marks SourceFunction interface as @deprecated.

We need to resolve all subtasks in FLINK-28045 and then deprecated SourceFunction interface.

Brief change log

reverts commit 07bf511

Verifying this change

This change is a revert 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): (yes)
• 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)

[leonardBang]

Thanks @ruanhang1993 for the work, LGTM

[flinkbot]

CI report:

• 1b03c7d Azure: SUCCESS

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[knaufk]

We need to resolve all subtasks in FLINK-28045 and then deprecated SourceFunction interface.

The vote on deprecating the SourceFunction explicitly stated, that we can deprecate it without having resolved all the subtasks. [1]

[1] https://lists.apache.org/thread/kv9rj3w2rmkb8jtss5bqffhw57or7v8v