Disable `Internal` visibility setting on GitLab.com
## Problem statement
The `Internal` project visibility can be misleading to users, especially on a public shared service like GitLab.com. Internal seems to convey some type of security, but on GitLab.com means anyone can access it, because sign ups are open.
## Proposed solution
Disable the `Internal` visibility setting on GitLab.com by [restricting this visibility setting in the admin panel](https://docs.gitlab.com/ee/public_access/public_access.html#restricting-the-use-of-public-or-internal-projects).
## Tested behavior
When enabled on an existing instance:
* New projects, groups, and snippets cannot be created with an `Internal` visibility setting. This option is greyed out:

* Existing projects, groups, and snippets with a visibility setting of `Internal` remain `Internal`. The visibility on these objects can only be modified to `Public` or `Private`.
* Attempting to create via the API fails with `"message": {"visibility_level": ["internal has been restricted by your GitLab administrator"]`.
* Admins can create projects, groups, and snippets with any visibility level, including `Internal`, via the UI or API.
issue