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

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

spec/components/middleware/anonymous_cache_spec.rb

@@ -143,7 +143,7 @@ describe Middleware::AnonymousCache::Helper do
 
       # depends on i7z implementation, but lets assume it is stable unless we discover
       # otherwise
-      expect($redis.get(helper.cache_key_body).length).to eq(16)
+      expect(Discourse.redis.get(helper.cache_key_body).length).to eq(16)
     end
 
     it "handles brotli switching" do

spec/components/middleware/request_tracker_spec.rb

@@ -264,7 +264,7 @@ describe Middleware::RequestTracker do
           User.where(id: -100).pluck(:id)
         end
         redis_calls.times do
-          $redis.get("x")
+          Discourse.redis.get("x")
         end
         result
       end