Enhancement: Automatically clear removed snapshots from cache

Previously, restic only removed snapshots from the cache on the host where the
`forget` command was executed. On other hosts that use the same repository, the
old snapshots remained in the cache.

Restic now automatically clears old snapshots from the local cache of the
current host.

https://github.com/restic/restic/issues/4934
https://github.com/restic/restic/pull/4981
