[keyserver] Run num admin change checks in updateRole only if intended role is not an admin

Summary:
Ran into an issue with running updateRole with a ghost commbot viewer to set the communityAdminID for createTaggedFarcasterCommunity causing a invalid_parameters as the changed admin count was equal to the current number of community admins.
This diff allows us to skip over this check if the request is to change members to admins

Depends on D13833

Test Plan: Was able to successfully update the role for the prospective comm user admin to an actual admin as confirmed by the iOS simualtor's chat list without encountering invalid_parameters.

Reviewers: ashoat, varun

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D13834