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/models/discourse_single_sign_on.rb

@@ -1,7 +1,5 @@
 # frozen_string_literal: true
 
-require_dependency 'single_sign_on'
-
 class DiscourseSingleSignOn < SingleSignOn
 
   class BlankExternalId < StandardError; end