Freeze the text variable string

This fixes Uploads Rewriter spec in Ruby 3
since interpolated strings are not frozen
by default anymore.

spec/lib/gitlab/gfm/uploads_rewriter_spec.rb

@@ -19,7 +19,7 @@
     end
 
     let(:text) do
-      "Text and #{image_uploader.markdown_link} and #{zip_uploader.markdown_link}"
+      "Text and #{image_uploader.markdown_link} and #{zip_uploader.markdown_link}".freeze # rubocop:disable Style/RedundantFreeze
     end
 
     def referenced_files(text, project)