target ns2.0 when ns1.x exists

[bruno-garcia]

The new Microsoft OSS library guidance recommends that when targeting .NET Standard 1.x, also add a .NET Standard 2.0 target.

✔️ DO include a netstandard2.0 target if you require a netstandard1.x target.
All platforms supporting .NET Standard 2.0 will use the netstandard2.0 target and benefit from having a smaller package graph while older platforms will still work and fall back to using the netstandard1.x target.