CHANGELOG-3.6: Add #20874 entry to 3.6.6

Signed-off-by: Ivan Valdes <ivan@vald.es>

Author: ivanvc

CHANGELOG/CHANGELOG-3.6.md

@@ -72,6 +72,7 @@ Previous change logs can be found at [CHANGELOG-3.5](https://github.com/etcd-io/
 - Fix [endpoint status not retuning the correct storage quota](https://github.com/etcd-io/etcd/pull/20790)
 - Fix [`--force-new-cluster can't clean up learners after creating snapshot`](https://github.com/etcd-io/etcd/pull/20896)
 - Fix [duplicate metrics collector registration that caused warning messages](https://github.com/etcd-io/etcd/pull/20905)
+- Fix [cannot promote member from follower when auth is enabled](https://github.com/etcd-io/etcd/pull/20874)
 
 ### Dependencies