Add an option to overwrite the existing checkpoint file

[solyaH]

What does this PR do?

If you want to preserve the checkpoint's file name, you can now specify override_existing=True on the ModelCheckpoint constructor. It will lead to overwriting the file path instead of writing a new file with the version appended.
Fixes #15106

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[carmocca]

Can you add a CHANGELOG.md entry, please?

[carmocca]

Awesome!