Declare steam sdk versions only once

Allar’s post on updating steamSDK

This is pure madness! On the wiki it tells you that you only have to change the version on OnlineSubsystemSteamPrivatePCH.h but you need to do the same thing at carious build.cs, ,at the SteamController.cpp, on the rules modules and much more! On the macOS build.cs it’s even hardcoded on the path!!! Can this mess be cleaned up? The optimal thing would be to declare the version number only once on a specific place, and this applies to all the other ThirdParty stuff as well!!