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.

plugins/poll/plugin.rb

@@ -300,8 +300,6 @@ after_initialize do
     end
   end
 
-  require_dependency "application_controller"
-
   class DiscoursePoll::PollsController < ::ApplicationController
     requires_plugin PLUGIN_NAME