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.

lib/discourse_redis.rb

@@ -3,7 +3,6 @@
 #
 #  A wrapper around redis that namespaces keys with the current site id
 #
-require_dependency 'cache'
 
 class DiscourseRedis
   class FallbackHandler