Introduce checkpoint migration

[awaelchli]

What does this PR do?

Part of #9396
Continues #8558

• When Lightning loads checkpoints, the migration gets applied automatically. Instructions are printed how to upgrade the checkpoint files permanently.
• This PR does not add any new migrations. These will be added in follow up PRs.
• The only migration function we have at the moment is the one that was in upgrade_checkpoint.py (refactored)
• A warning is raised when attempting to load a checkpoint that is newer than the current Lightning version. This is optional, but I added this to signal "migration" here means we only support going forward in time, not back. Please do not ask me to add support for going back in time.

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Did you have fun?

I made sure I had fun coding 🙃

cc @Borda @awaelchli @ananthsub @ninginthecloud @rohitgr7 @otaj