Compatibility with security.enable_super_user = false

Problem/Motivation

With this setting disabled, UID 1 is not special anymore: https://www.drupal.org/node/2910500 →
Now any user can be an administrator.

Steps to reproduce

Add this to services.yml:

parameters:
  security.enable_super_user: false

Browse to /admin/people/role-settings and select a role as the admin role.
Now any user with this role has every permission.

Proposed resolution

The module should disable editing any user with the admin role.