Add Rainbow in gems/gitlab-backup-cli/lib/gitlab/backup/cli/output

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/159710



Merged-by: Peter Leitzen <pleitzen@gitlab.com>
Approved-by: Peter Leitzen <pleitzen@gitlab.com>
Co-authored-by: Martin Wortschack <mwortschack@gitlab.com>