Finalize backfill_approval_merge_request_rules_users_project_id BBM

What does this MR do and why?

Finalize backfill_approval_merge_request_rules_users_project_id BBM

Addresses &11670

Following the re-queue and completion of !197054 (merged). We now look to finalize this batch background migration (BBM).

• BBM completed successfully

  

• 0 records approval_merge_request_rules_users#project_id IS NULL on gitlab.com -> https://postgres.ai/console/gitlab/gitlab-production-main/sessions/41965/commands/128835

• Can finalize BBM in milestone >= 18.3 since the BBM was merged in 18.2 (last required stop): https://docs.gitlab.com/development/database/required_stops/

  

References

• &11670
• !197054 (merged)

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