MR 1/2 - Prevent admins from deleting users that are sole owner of organization

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/171345



Merged-by: Olena Horal-Koretska <ohoralkoretska@gitlab.com>
Approved-by: Mike Nichols <mnichols@gitlab.com>
Approved-by: Ammar Alakkad <aalakkad@gitlab.com>
Approved-by: Sam Figueroa <sfigueroa@gitlab.com>
Approved-by: Vijay Hawoldar <vhawoldar@gitlab.com>
Approved-by: Olena Horal-Koretska <ohoralkoretska@gitlab.com>
Reviewed-by: Ammar Alakkad <aalakkad@gitlab.com>
Reviewed-by: Vijay Hawoldar <vhawoldar@gitlab.com>
Co-authored-by: Peter Hegman <phegman@gitlab.com>