Restrict license removal for GitLab.com (SaaS)

Proposal

Prevent the removal of the GitLab.com license in the admin UI (or show a warning dialog/message) as this will remove all Premium/Ultimate features for all SaaS customers.

This could be a simple check and error message when clicking on the removal button - if GitLab.com? that this change needs to go through a different route (console, API...) and perhaps document it in an issue.

Another option could be to completely remove the button if GitLab.com?.

This is a corrective action from gitlab-com/gl-infra/production#7087 (comment 950682267)