chore: Change primary actor to Conversation for all the notification types. (#8435)

app/jobs/migration/remove_message_notifications.rb

@@ -0,0 +1,8 @@
+# Delete migration and spec after 2 consecutive releases.
+class Migration::RemoveMessageNotifications < ApplicationJob
+  queue_as :scheduled_jobs
+
+  def perform
+    Notification.where(primary_actor_type: 'Message').in_batches(of: 100).each_record(&:destroy)
+  end
+end