Fix CHANGELOG double-update

[ci skip]

Signed-off-by: Rémy Coutable <remy@rymai.me>

Former-commit-id: bc16d767

CHANGELOG

 Please view this file on the master branch, on stable branches it's out of date.
-v 8.13.0 (unreleased)
-  - Speed-up group milestones show page
 v 8.13.0 (unreleased)
+  - Speed-up group milestones show page
 v 8.12.1 (unreleased)