Enhancement: Keep oldest snapshot when there aren't enough snapshots

The `forget` command does not preserve the oldest snapshot incase the 
keep-* parameters are not satisfied, which led to users not being able to
preserve old data. Now, restic will always preserve the oldest snapshot 
whenever any of the keep-* options to the `forget` command are not 
satisfied.


https://github.com/restic/restic/issues/3624
https://github.com/restic/restic/pull/4366
https://forum.restic.net/t/keeping-yearly-snapshots-policy-when-backup-began-during-the-year/4670/2