Draft: Finalize the backfill user details migration
What does this MR do and why?
Finalize the backfill user details migration
- after required stop of 17.5 as per docs
- see https://docs.gitlab.com/ee/development/database/batched_background_migrations.html#finalize-a-batched-background-migration
- see #462918
Changelog: other
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Related to #462918