Change: Prune small packfiles more aggressively

The `prune` command now repacks more small packfiles by default. The option
`--repack-small` is no longer needed and has been marked as deprecated. The
`--repack-smaller-than` option can still be used to further control repacking
of small pack files.

https://github.com/restic/restic/issues/5293
https://github.com/restic/restic/pull/21803
