Add batched background migration to backfill namespace_id on notes

What does this MR do and why?

Backfills namespace_id onto notes records in order to support sharding.

Related to #479909 (closed)

Database

main: == [advisory_lock_connection] object_id: 128240, pg_backend_pid: 99238
main: == 20240822220027 QueueBackfillMissingNamespaceIdOnNotes: migrating ===========
main: == 20240822220027 QueueBackfillMissingNamespaceIdOnNotes: migrated (0.0498s) ==

main: == [advisory_lock_connection] object_id: 128240, pg_backend_pid: 99238
ci: == [advisory_lock_connection] object_id: 128800, pg_backend_pid: 99240
ci: == 20240822220027 QueueBackfillMissingNamespaceIdOnNotes: migrating ===========
ci: -- The migration is skipped since it modifies the schemas: [:gitlab_main].
ci: -- This database can only apply migrations in one of the following schemas: [:gitlab_ci, :gitlab_internal, :gitlab_shared].
ci: == 20240822220027 QueueBackfillMissingNamespaceIdOnNotes: migrated (0.0074s) ==

ci: == [advisory_lock_connection] object_id: 128800, pg_backend_pid: 99240