design and implement backup strategy for MinIO buckets or the entire server

We're considering using MinIO for more and more things, mainly GitLab (artifacts storage in #41403 and gitaly backups in #40518) but possibly other (e.g. metrics storage in tpo/network-health/metrics/collector#40023 (closed)).

Right now, we don't have any backups of that server, which is probably fine: we only store container images there, which can be regenerated in case of a catastrophe. But if we start storing gitaly backups and gitlab artifacts, it needs to be permanent now.

Research how backups can be performed, develop a policy and implement it.

Next steps:

• research articles anarcat found on the topic (see wallabag)
• discuss the idea in the network
• decide if we want this per bucket or per site
• write up a proposal
• implement proposal
• document and test backup/restore procedures