Make Microsoft.CSharp a conditional package based on TFM

[James Thompson]

Is your feature request related to a problem? Please describe.

I want to minimise dependencies in my project by utilising framework dependencies wherever possible

Describe the solution you'd like

A new TFM is added net Standard 2.1 &/or NET 6
The Microsoft.CSharp package becomes conditional so only added for the older frameworks

Describe alternatives you've considered

Accept the additional dependency

Additional context

n/a