Update CHANGELOG

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CHANGELOG

@@ -128,6 +128,7 @@ v 8.12.0 (unreleased)
   - Add notification_settings API calls !5632 (mahcsig)
   - Remove duplication between project builds and admin builds view !5680 (Katarzyna Kobierska Ula Budziszewska)
   - Deleting source project with existing fork link will close all related merge requests !6177 (Katarzyna Kobierska Ula Budziszeska)
+  - Return 204 instead of 404 for /ci/api/v1/builds/register.json if no builds are scheduled for a runner !6225
 
 v 8.11.6 (unreleased)
   - Fix an error where we were unable to create a CommitStatus for running state