Move to a single .NET Standard 2.0 target

[akamsteeg]

Currently, Pitcher multi-targets to .NET 4.0 and .NET Standard 1.0. These are both very old and not very likely to be used for the modern high-performance code Pitcher is intended to be used in.

Suggestion: Drop the .NET 4.0 target and move to .NET Standard 2.0 exclusively.

.NET Standard 2.0 is properly supported from .NET 4.7.2 onwards (if we forget the dumpster fire that's the tacked on support in 4.6.something). That means we support everything from april 2018 onwards. That's good enough I think And, moving to a single modern TFM should make future upgrades and additions easier too.

This is a breaking change, we need to increase the major version number for this.