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/favorite_group.rb

@@ -174,7 +174,7 @@ class FavoriteGroup < ApplicationRecord
       return if contains?(post_id)
 
       clear_post_id_array
-      update_attributes(:post_ids => add_number_to_string(post_id, post_ids))
+      update(post_ids: add_number_to_string(post_id, post_ids))
     end
   end
 
@@ -184,7 +184,7 @@ class FavoriteGroup < ApplicationRecord
       return unless contains?(post_id)
 
       clear_post_id_array
-      update_attributes(:post_ids => remove_number_from_string(post_id, post_ids))
+      update(post_ids: remove_number_from_string(post_id, post_ids))
     end
   end