Deprecate ModelCheckpoint.save_checkpoint

[carmocca]

What does this PR do?

Fixes #12418 (comment)

This is a callback so saving should be entirely managed by hooks.

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

None

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cc @Borda @tchaton @rohitgr7 @carmocca @awaelchli @ninginthecloud @jjenniferdai