Add a note about ES update to the GitLab update guide

doc/update/8.17-to-9.0.md

@@ -301,6 +301,12 @@ sudo -u git -H bundle exec rake gitlab:check RAILS_ENV=production
 
 If all items are green, then congratulations, the upgrade is complete!
 
+### 12. Elasticsearch server update (if you currently use Elasticsearch)
+
+In 9.0 release we bumped the required version of Elasticsearch from 2.4.x to 5.1.x.
+Please update it following the official [documentation](https://www.elastic.co/guide/en/elasticsearch/reference/5.1/setup-upgrade.html). Indexes created by Elasticsearch 2.4.x can be read by Elasticsearch 5.1.x.
+
+
 ## Things went south? Revert to previous version (8.17)
 
 ### 1. Revert the code to the previous version