Improve background migration helpers

What does this MR do and why?

Update helpers to process background migrations with different gitlab_schema when it is compatible with the connection.

This will resolve issues with migrations not being found when table's gitlab_schema change while migration is in progress.

#503461

References

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MR acceptance checklist

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How to set up and validate locally

1. Create migration with restrict_gitlab_migration gitlab_schema: :gitlab_main to enqueue background migration.
2. Create another migration with restrict_gitlab_migration gitlab_schema: :gitlab_main_cell to finalize it.
3. Run DBLAB_ENVIRONMENT=true be rails db:migrate
   • On master this will fail with

     StandardError: An error has occurred, all later migrations canceled:
     
     called ensure_batched_background_migration_is_finished with non-existent migration name

   • On this branch it will succeed and the migration will be finalized.

Related to #503461