Improve gitlab-backup
We currently drop a tar.gz in /root/backup-gitlab of the registry, uploads etc. and then borg-backup this directory. Creating the archive obviously takes some time, so can't we drop the .gz compression or just dump a directory, as borg will create a compressed archive anyway (with zstd). We also lack concurrency, so it now backs up repos 1 by 1.
https://docs.gitlab.com/ee/raketasks/backup_restore.html#back-up-git-repositories-concurrently