Cleaned most of the thirdparty folder from foreign binaries

A followup of https://github.com/OpenRA/OpenRA/pull/7190 that fetches all I could find from www.nuget.org.

As a side-effect it updates Mono.Nat with an improved version from https://github.com/nterry/Mono.Nat and it also updates SharpFont to the latest version.
I'm not sure if we should really remove all these. We're still shipping and distributing most of those libs. Maybe move this whole file to the readme, or a separate licensing document? NUnit and StyleCop can be left out, they are just build dependencies and don't get distributed.