Change: prune small packfiles more aggressively

The `prune` command now repacks more small packfiles by default. The option `--repack-small`
is not longer needed and has been marked as deprecated. You can still use `--repack-smaller-than`
to further control repacking of small pack files.

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