Update CHANGELOG

CHANGELOG

@@ -28,6 +28,7 @@ v 8.8.0 (unreleased)
   - API support for the 'since' and 'until' operators on commit requests (Paco Guzman)
   - Fix Gravatar hint in user profile when Gravatar is disabled. !3988 (Artem Sidorenko)
   - Expire repository exists? and has_visible_content? caches after a push if necessary
+  - Fix unintentional filtering bug in issues sorted by milestone due (Takuya Noguchi)
 
 v 8.7.3
   - Emails, Gitlab::Email::Message, Gitlab::Diff, and Premailer::Adapter::Nokogiri are now instrumented