DEV: Update rubocop-discourse to latest version

The lastest version of rubocop-discourse enables rules regarding
plugins.

spec/requests/users_controller_spec.rb

@@ -4903,7 +4903,7 @@ RSpec.describe UsersController do
   end
 
   describe "#search_users" do
-    fab!(:topic) { Fabricate :topic }
+    fab!(:topic)
     let(:user) { Fabricate :user, username: "joecabot", name: "Lawrence Tierney" }
     let(:post1) { Fabricate(:post, user: user, topic: topic) }
     let(:staged_user) { Fabricate(:user, staged: true) }