Add cron job for BatchedBackgroundMigrationWorker on self-managed instances

We have not enabled the cron job for BatchedBackgroundMigrationWorker on self-managed instances in %14.0, which results to the error mentioned in gitlab-org/charts/gitlab#2785 (closed) when an instance upgrades to a higher (minor) version that includes a column swap migration (e.g. !64577 (merged))

We want to add the cron job and also backport it to %14.0, which is the version that the batched background migrations for backfilling the new bigint columns were introduced.

For more info check the following:

• #288005 (comment 614744621)
• gitlab-org/charts/gitlab#2785 (closed)