Rename callbacks/base.py to callbacks/callback.py

[shantam-8]

What does this PR do?

Renames callbacks/base.py to callbacks/callback.py and changes other instances in different parts of the repository. Also changes callbacks/base.py to provide a deprecation warning.

Fixes #11973

IMPORTANT: This PR needs to be "Rebase and Merge"d. Do not add merge commits!

Does your PR introduce any breaking changes? If yes, please list them.

None

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[awaelchli]

@rohitgr7 could you rebase once more? Thx