[keyserver] Prevent the only admin role from being changed on the keyserver

Summary:
Although we will handle this on the client-side, we should also make sure the keyserver supports the use case where we throw a server error if the sole admin of a community tries to change their own role.

Depends on D8140

Test Plan:
Please see the video below

Reviewers: ginsu, ashoat

Reviewed By: ashoat

Subscribers: atul, ashoat, tomek

Differential Revision: https://phab.comm.dev/D8200