[AIRFLOW-6657] Deprecate BaseBranchOperator

[yuqian90]

Everything that can be done by deriving from BaseBranchOperator can also be done using BranchPythonOperator with more concise and readable code.

E.g. LatestOnlyOperator can be implemented from BranchPythonOperator easily. The test tasks in test_branch_operator.py are also better implemented using BranchPythonOperator like the ones in tests/operators/test_python.py

The changes in this PR to LatestOnlyOperator does not require additional testing because there's already a test_latest_only_operator.py that works fine after the change.

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Issue link: AIRFLOW-6657

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[ashb]

Did we make any decisions on if we need this or not? I remember there was a chat somewhere after you opened this PR.

[yuqian90]

Did we make any decisions on if we need this or not? I remember there was a chat somewhere after you opened this PR.

Hi @ashb I did post the PR in #development but I don't think people replied back then. But you are right we should discuss if BaseBranchOperator should be deprecated.

I read the discussion history on #5231. My understanding is @chronitis originally added BaseBranchOperator when most of the skipping logic lived inside BranchPythonOperator itself. He then went on to refactor BranchPythonOperator and SkipMixin. He did a good job refactoring them till a point where almost all of the logic now lives in SkipMixin.skip_all_except() itself, making it much more reusable. So it rendered BaseBranchOperator much less useful (because people can use SkipMixin directly if they wish).

Also as I pointed out, whatever we can do with BaseBranchOperator can also be done with BranchPythonOperator. That's why i find it unnecessary to have a separate class.

@chronitis @ashb please let me know what you think.

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