CVE-2024-41942: JupyterHub has a privilege escalation vulnerability with the `admin:users` scope

### Summary If a user is granted the `admin:users` scope, they may escalate their own privileges by making themselves a full admin user. ### Details The `admin:users` scope allows a user to edit user records: > admin:users > > Read, write, create and delete users and their authentication state, not including their servers or tokens. > > -- https://jupyterhub.readthedocs.io/en/stable/rbac/scopes.html#available-scopes However, this includes making users admins. Admin users are granted scopes beyond `admin:users` making this a mechanism by which granted scopes may be escalated. ### Impact The impact is relatively small in that `admin:users` is already an extremely privileged scope only granted to trusted users. In effect, `admin:users` is equivalent to `admin=True`, which is not intended. Note that the change here only prevents escalation to the built-in JupyterHub admin role that has unrestricted permissions. It does not prevent users with e.g. `groups` permissions from granting themselves or other users permissions via group membership, which is intentional.