.net Standard 2.0 instead of 2.1

[lutz]

It will be great if your library would use the .net Standard 2.0 instead of 2.1 to support the compatibility with .net framework 4.8.

I checkout your code and it seems to be that the only thing is to do this of adding the nuget reference to "Microsoft.Bcl.HashCode" (link) for your call of HashCode.COmbine()

The rest is compatible

[AngusJohnson]

I'm not against using .NET Standard 2.0, but I'm not keen on dependencies on other packages.

[s-costa]

I, too, suggest switching to .net Standard 2.0 instead of 2.1.
Currently all projects that use .Net Framework, cannot use Clipper2 for this reason.
It has been discussed before:
#225

[lutz]

I`ve create a PR for this.