Replace deprecated update_attributes with update.

https://rubyinrails.com/2019/04/09/rails-6-1-activerecord-deprecates-update-attributes-methods/

DEPRECATION WARNING: update_attributes! is deprecated and will be removed from Rails 6.1 (please, use update! instead)

app/models/forum_topic.rb

@@ -193,7 +193,7 @@ class ForumTopic < ApplicationRecord
 
   def merge(topic)
     ForumPost.where(:id => self.posts.map(&:id)).update_all(:topic_id => topic.id)
-    topic.update_attributes(:response_count => topic.response_count + self.posts.length, :updater_id => CurrentUser.id)
+    topic.update(response_count: topic.response_count + self.posts.length, updater_id: CurrentUser.id)
     self.update_columns(:response_count => 0, :is_deleted => true, :updater_id => CurrentUser.id)
   end