Finalize backfilling ci mirror tables

What does this MR do and why?

This MR will make us sure that backfilling is completed in self-hosted instances.

As per !79221 (comment 822293189);

This means that the BG migration is already done on self-hosted or will be done. As a cleanup, we can call the finalize_background_migration helper to process all pending migrations inline which will be a no-op on GL.com.

Related to #346786

Database

Up

== 20220127132200 CleanupBackfillCiNamespaceMirrors: migrating ================
== 20220127132200 CleanupBackfillCiNamespaceMirrors: migrated (0.1884s) =======

== 20220127132201 CleanupBackfillCiProjectMirrors: migrating ==================
== 20220127132201 CleanupBackfillCiProjectMirrors: migrated (0.2245s) =========

Down

== 20220127132201 CleanupBackfillCiProjectMirrors: reverting ==================
== 20220127132201 CleanupBackfillCiProjectMirrors: reverted (0.0000s) =========

== 20220127132200 CleanupBackfillCiNamespaceMirrors: reverting ================
== 20220127132200 CleanupBackfillCiNamespaceMirrors: reverted (0.0000s) =======

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