FIX: Ruby 3 does not freeze interpolated string (#14567)

Ruby 2.7 or earlier `+contents` returns self.dup
when `frozen_string_literal: true`. However, Ruby 3.0 returns self
because this string is interpolated one, which is not frozen anymore.

This commit uses self.dup to return duplicated string regardless Ruby
versions.
https://bugs.ruby-lang.org/issues/17104

app/models/topic_embed.rb

@@ -34,7 +34,7 @@ class TopicEmbed < ActiveRecord::Base
       contents = first_paragraph_from(contents)
     end
     contents ||= ''
-    contents = +contents << imported_from_html(url)
+    contents = contents.dup << imported_from_html(url)
 
     url = normalize_url(url)