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 as well as NUnit 2 to the latest version.