Backup sync operator should cleanup MedusaBackupJob objects

[adejanovski]

The sync MedusaTask deletes MedusaBackup objects that don't have an actual backup in the storage bucket.
But it leaves the MedusaBackupJob objects, which can pile up a lot over time.
We'd need to make them part of the cleanup, deleting the MedusaBackupJobs for backups that have been purged from the storage bucket. This should only happen if the MedusaBackupJob is finished already (may it be successful or not). Pending/Running jobs should be left untouched.

Definition of Done

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1. MedusaBackupJobs are deleted during a sync MedusaTask if no corresponding backup exists in the backend and if they're in a finished state already.

[adejanovski]

This was done in #1347