Drop support for .NET Framework 4.0 - 4.6 (e.g. require 4.6.1, which supports .NET Standard 2.0) #1299

I propose we drop support for the older .NET Framework versions, that do not support .NET Standard 2.0. According to the official documentation this would leave us with:

• .NET Framework 4.6.1+ (requires manual installation on Windows 8.X, Windows 10 1507 (the very first one), and Windows Server 2012 + R2)
• .NET Core 2.0+
• Mono 5.4+
• Xamarin.Mac 3.8+
• Unity 2018.1+

This should remove the xplat option, and a few polyfills as well as dependency on Python modules inside C# code in some cases (OS detection).

@filmor @amos402 @benoithudson @BadSingleton