Update CHANGELOG

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CHANGELOG

 Please view this file on the master branch, on stable branches it's out of date.
 
-v 8.4.2 (unreleased)
+v 8.4.2
   - Bump required gitlab-workhorse version to bring in a fix for missing
     artifacts in the build artifacts browser
   - Get rid of those ugly borders on the file tree view
@@ -9,9 +9,7 @@ v 8.4.2 (unreleased)
     improvement when checking if a repository was empty
   - Add instrumentation for Gitlab::Git::Repository instance methods so we can
     track them in Performance Monitoring.
-  - Increase contrast between highlighted code comments and inline diff marker.
-
-v 8.4.2 (unreleased)
+  - Increase contrast between highlighted code comments and inline diff marker
   - Fix method undefined when using external commit status in builds
 
 v 8.4.1