Finalize BackfillOrganizationIdKeys batched background migration

What does this MR do and why?

Finalize the BackfillOrganizationIdKeys batched background migration.

The BBM was enqueued and completed on GitLab.com in 18.7: #577245 (closed)

According to the docs that means it is safe to finalize in milestone 18.9, which is one milestone after the next required stop (18.8).

Migration output

Up

% gdk rails db:migrate:up:main VERSION=20260122161141

main: == [advisory_lock_connection] object_id: 133300, pg_backend_pid: 83931
main: == 20260122161141 FinalizeBackfillOrganizationIdKeys: migrating ===============
main: -- transaction_open?(nil)
main:    -> 0.0000s
main: == 20260122161141 FinalizeBackfillOrganizationIdKeys: migrated (0.1874s) ======

main: == [advisory_lock_connection] object_id: 133300, pg_backend_pid: 83931

Down

% gdk rails db:migrate:down:main VERSION=20260122161141

main: == [advisory_lock_connection] object_id: 133280, pg_backend_pid: 84850
main: == 20260122161141 FinalizeBackfillOrganizationIdKeys: reverting ===============
main: == 20260122161141 FinalizeBackfillOrganizationIdKeys: reverted (0.0245s) ======

main: == [advisory_lock_connection] object_id: 133280, pg_backend_pid: 84850

References

• Issue: #577246
• Backfill issue: #577245 (closed)
• Backfill MR: !210115 (merged)

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Related to #577246