Enhancement: `restic repair snapshots --forget` removes broken snapshots

In the past restic had no command to remove broken snapshot files.

`restic repair snapshots --forget` can now remove broken snapshot files,
which have been found previously by `restic check` or by earlier error messages from restic commands.

For example, `restic repair snapshots --forget 1d204771` will remove a broken snapshot file with id `1d204771`.

https://github.com/restic/restic/issues/21892
https://github.com/restic/restic/pull/21907