Drop .NET 4.0 Build Target

[rprouse]

.NET 4.0 has been EOL for around five years, so I'd like to drop support for it. This would allow us to clean up our async code.

[stevenaw]

I like this idea and am all for simplifying the code base.

One question: Would this be considered a breaking change (or can it be released in NUnit 3.x ?). Five years EOL is quite a while, I can't imagine many people are still targeting Framework 4.0