Add 8.6.4 changelog item

[ci skip]
Signed-off-by: Rémy Coutable <remy@rymai.me>

CHANGELOG-EE

@@ -4,6 +4,9 @@ v 8.7.0 (unreleased)
   - Update GitLab Pages to 0.2.1: support user-defined 404 pages
   - Refactor group sync to pull access level logic to its own class. !306
 
+v 8.6.4
+  - No EE-specific changes
+
 v 8.6.3
   - Fix other cases where git hooks would fail due to old commits. !310
   - Exit ElasticIndexerWorker's job happily if record cannot be found. !311