Update Azure installation documentation

https://docs.gitlab.com/ee/install/azure/

This doc walks you through installing on Azure via a Marketplace image, which is still at version 8.6.5-ee (very old). It's not even reasonable to install the old version and then run a bunch of updates.

We should either remove this doc, or work with Distribution to update and maintain the image.

An alternate version of this guide could describe installing a Linux VM and then installing Omnibus on that.