Move batched BG helpers to separate module

What does this MR do and why?

Move migration helpers for batched background migrations into a separate module from the existing background migrations, to clearly indicate they are two separate and incompatible approaches. Also added some comments on the module to explain what batched background migrations are, and their current status.

We've had some confusion around this recently which was brought up in !73847 (merged), and it's not clear how these different methods work together.

MR acceptance checklist

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