Move finalize background migration to post-deploy

What does this MR do and why?

This MR moves the migration added in !92119 (merged) to a post-deployment migration. See this comment for full details on why.

Database

Migration up output:

main: == 20220713133515 CleanupBackfillDraftStatusesOnMergeRequests: migrating ======
main: -- transaction_open?()
main:    -> 0.0000s
main: == 20220713133515 CleanupBackfillDraftStatusesOnMergeRequests: migrated (0.0295s)

ci: == 20220713133515 CleanupBackfillDraftStatusesOnMergeRequests: migrating ======
ci: -- transaction_open?()
ci:    -> 0.0000s
ci: == 20220713133515 CleanupBackfillDraftStatusesOnMergeRequests: migrated (0.0088s)

Migration down output:

ci: == 20220713133515 CleanupBackfillDraftStatusesOnMergeRequests: reverting ======
ci: == 20220713133515 CleanupBackfillDraftStatusesOnMergeRequests: reverted (0.0007s)

main: == 20220713133515 CleanupBackfillDraftStatusesOnMergeRequests: reverting ======
main: == 20220713133515 CleanupBackfillDraftStatusesOnMergeRequests: reverted (0.0006s)

Screenshots or screen recordings

N/A

How to set up and validate locally

N/A

MR acceptance checklist

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