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/session_controller.rb

@@ -1,11 +1,5 @@
 # frozen_string_literal: true
 
-require_dependency 'rate_limiter'
-require_dependency 'single_sign_on'
-require_dependency 'single_sign_on_provider'
-require_dependency 'url_helper'
-require_dependency 'webauthn/webauthn'
-
 class SessionController < ApplicationController
   class LocalLoginNotAllowed < StandardError; end
   rescue_from LocalLoginNotAllowed do