[SM - Iteration 1] Don't allow Guest users to move themselves into a paid seat

Problem to solve

Currently, a Guest user can move themselves into a paid seat by creating a project in their personal namespace and adding themselves to a role higher than Guest on that project.

This means that admins have less visibility and control over whether or not a user takes up a paid seat.

https://docs.gitlab.com/ee/user/permissions.html#free-guest-users-ultimate

Designs

None needed for iteration 1 but here for reference #419645 (closed)

Proposals

Recommendation: Iteration 1+ Iteration 2 -- Option A (or B pending outcome of data) but open to all pathways:

Question	Iteration 1
Audience	Current SM customers who have told us this (guests promoting themselves to a paid seat) is a problem
Solution	Tell this audience to make these guests external
What we need to pursue this solution	PoC from engg to confirm this solution is viable and how it works ✅ . See PoC link below.
Downside	Requires manual work from the customer side to make the users external & will also require re-sharing internal/private projects. Won't solve the problem for all customers but only those who have expressed pain around the problem.
Upside	This is already a viable solution.
UX	None needed
Outcome of this issue -- Result of PoC	#415616 (comment 1572334036)
Next Steps	Update documentation to indicate this as a solution #426298 (closed)