Finalize EnableRestrictUserDefinedVariables migration

Avielle Wolferequested to merge
521161-finalize-EnableRestrictUserDefinedVariables into master

What does this MR do and why?

The 17.11 required stop ran the queued migration, so we now need to finalize it.

Finalizing batched background migrations docs (for easy reference): https://docs.gitlab.com/development/database/batched_background_migrations/#finalize-a-batched-background-migration

Issue: Finalize background migration EnableRestrictUse... (#521161 - closed)

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