Implement multi-level pruning scheme of backups
We currently use the basic built-in pruning of velero, with a ttl of all backups set to 60 days.
This should be refined/extended, to have for example
- daily backups for two weeks;
- weekly backups for a month;
- monthly backups for a year (?).
Numbers are easy enough to change later of course. However, they do tie in to the issue of old backups being possibly hard to restore from, because velero has insisted so far on restoring the software (app pod) together with the data, which can become problematic in the face of e.g. nextcloud that doesn't allow skipping over major versions.