DEV: Upgrading Discourse to Zeitwerk (#8098)

Zeitwerk simplifies working with dependencies in dev and makes it easier reloading class chains. 

We no longer need to use Rails "require_dependency" anywhere and instead can just use standard 
Ruby patterns to require files.

This is a far reaching change and we expect some followups here.

app/controllers/admin/users_controller.rb

@@ -1,9 +1,5 @@
 # frozen_string_literal: true
 
-require_dependency 'user_destroyer'
-require_dependency 'admin_user_index_query'
-require_dependency 'admin_confirmation'
-
 class Admin::UsersController < Admin::AdminController
 
   before_action :fetch_user, only: [:suspend,