rotate gitlab backups using normal procedure

I just found out there's actually already a routing in GitLab itself to rotate backups, in:

https://docs.gitlab.com/ee/raketasks/backup_restore.html#limit-backup-lifetime-for-local-files-prune-old-backups

It seems that this settings purges backups older than 7 days, for example:

## Limit backup lifetime to 7 days - 604800 seconds
gitlab_rails['backup_keep_time'] = 604800

Now it wouldn't flush away our secrets backups, but i figured those we should backup and manage some other ways anyways (ie. with bacula).

I just dealt with an outage tonight because the disk was full again so i figured I would look into backups. ;)