Checkpoint migration for loop's internal state

[awaelchli]

What does this PR do?

The migration tools introduced in #15237 allow us to get rid of some code in the loops and checkpoint connector, as we can directly define migrations that apply the necessary changes to the checkpoint dict.

This also has the benefit that the checkpoint can be permanently upgraded (see #15333)

Part of #9396

Relevant old PRs:

• Integrate global step with progress tracking #11805
• Use global_step while restoring logging step for old checkpoints #13645

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