Add GitLabDuo feedback string for diff review comments

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/184073



Merged-by: Phil Hughes <me@iamphill.com>
Approved-by: Phil Hughes <me@iamphill.com>
Reviewed-by: Phil Hughes <me@iamphill.com>
Co-authored-by: Kinshuk Singh <kinsingh@gitlab.com>