DEV: s/\$redis/Discourse\.redis (#8431)

This commit also adds a rubocop rule to prevent global variables.

spec/tasks/redis_spec.rb

@@ -3,7 +3,7 @@
 require 'rails_helper'
 
 RSpec.describe "Redis rake tasks" do
-  let(:redis) { $redis.without_namespace }
+  let(:redis) { Discourse.redis.without_namespace }
 
   before do
     @multisite = Rails.configuration.multisite