Change: Include full key ID in JSON output of `key list`

We have changed the JSON output of the `key list` command to include the full
key ID instead of just a shortened version, as the latter can be ambiguous
in some rare cases. To derive the short ID, please truncate the full ID down to
eight characters.

https://github.com/restic/restic/issues/4744
https://github.com/restic/restic/pull/4745
