Remove 8.4 unreleased note from changelog

CHANGELOG

@@ -5,7 +5,7 @@ v 8.5.0 (unreleased)
   - Upgrade gitlab_git to 7.2.23 to fix commit message mentions in first branch push
   - New UI for pagination
 
-v 8.4.0 (unreleased)
+v 8.4.0
   - Allow LDAP users to change their email if it was not set by the LDAP server
   - Ensure Gravatar host looks like an actual host
   - Consider re-assign as a mention from a notification point of view