set_attribute_was is deprecated in Rails 6, so skipped it.

Ref. mbleigh#955

lib/acts_as_taggable_on/taggable/core.rb

@@ -50,7 +50,7 @@ def #{tag_type}_list=(new_tags)
               parsed_new_list = ActsAsTaggableOn.default_parser.new(new_tags).parse
 
               if self.class.preserve_tag_order? || parsed_new_list.sort != #{tag_type}_list.sort
-                set_attribute_was('#{tag_type}_list', #{tag_type}_list)
+                # set_attribute_was('#{tag_type}_list', #{tag_type}_list)
                 write_attribute("#{tag_type}_list", parsed_new_list)
               end
 
@@ -314,4 +314,3 @@ def find_or_create_tags_from_list_with_context(tag_list, _context)
     end
   end
 end
-