WAL-G's backup-fetch .. LATEST cannot be considered reliable anymore – mitigation is needed

`backup-fetch .. LATEST" cannot be considered as reliable if we touch old backups and change modification time (currently we do, see the thread here: production#2297 (comment 364176758) and production#2297 (comment 368316725)).

TODO:

• raise this problem in WAL-G (it would be good to switch to either creation time or LSNs) → https://github.com/wal-g/wal-g/issues/694
• prepare a fix for backup verification jobs which are suffering from broken "LATEST" behavior (an MR in "gitlab-restore") → https://ops.gitlab.net/gitlab-com/gl-infra/gitlab-restore/postgres-gprd/-/merge_requests/44
• adjust backup runbooks to avoid using backup-fetch .. LATEST in the case of DR (❗ we might be in a big trouble if we need to restore from backups ASAP, and "LATEST" gives us not the fresh backup)