[docs] document on how to correctly upgrade the Fleet Server Agent on premise

[toby-sutor]

It seems like there is no clear documentation available about how to correctly the upgrade the Fleet Server in on-prem environments. This frequently leads to confusion and frustration.

It is assumed, that the correct way is to follow the steps for upgrading the standalone Elastic Agent. However, since the Fleet Server itself is visible in the Fleet UI, this appears to be a counterintuitive assumption.

The steps for upgrading Fleet managed Agents are very elaborative but do not cover this topic either. Given the restriction:

Elastic Agent cannot be upgraded to a version higher than the highest currently installed version of Fleet Server. 
When you upgrade a set of Elastic Agents that are currently at the same version, 
you should first upgrade any agents that are acting as Fleet Server (any agents that have a Fleet Server policy associated with them).

one could conclude, that the Fleet Server cannot be upgraded through the Fleet UI.

As such, it would be great if we could either get a new page next to https://www.elastic.co/guide/en/fleet/8.17/upgrade-elastic-agent.html, or expand this page with the correct steps on how to upgrade the Fleet Server in a supported manner for Linux, Windows and containers. Thank you.