update changelog

CHANGELOG-EE

 Please view this file on the master branch, on stable branches it's out of date.
 
-v 8.6.0
-  - [Elastic] Update index on push to wiki
-
-v 8.5.0 (unreleased)
 v 8.6.0 (unreleased)
   - Improve weight filter for issues
   - [Elastic] Add elastic checker to gitlab:check
   - [Elastic] Added UPDATE_INDEX option to rake task
+  - [Elastic] Update index on push to wiki
 
 v 8.5.1
   - Fix adding pages domain to projects in groups