Add better deprecation notice for the upgrader. Too easy to miss otherwise...

Add better deprecation notice for the upgrader. Too easy to miss otherwise https://twitter.com/0x663030623472/status/646262552796594176

doc/update/upgrader.md

-# GitLab Upgrader
+# GitLab Upgrader (deprecated)
 *DEPRECATED* We recommend to [switch to the Omnibus package and repository server](https://about.gitlab.com/update/) instead of using this script.