DEV: Update rubocop-discourse to latest version

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

plugins/discourse-narrative-bot/lib/discourse_narrative_bot/post_guardian_extension.rb

@@ -0,0 +1,21 @@
+# frozen_string_literal: true
+
+module DiscourseNarrativeBot
+  module PostGuardianExtension
+    extend ActiveSupport::Concern
+
+    prepended do
+      alias_method :existing_can_create_post?, :can_create_post?
+
+      def can_create_post?(parent)
+        if SiteSetting.discourse_narrative_bot_enabled &&
+             parent.try(:subtype) == "system_message" &&
+             parent.try(:user) == ::DiscourseNarrativeBot::Base.new.discobot_user
+          return true
+        end
+
+        existing_can_create_post?(parent)
+      end
+    end
+  end
+end