Change request: C# Target .NET Standard 2.0 to keep compatibility with .NET Framework

[NicholasZiglioCaracol]

I have seen that since the last NuGet package release, the C# port which has significant bug fixes is now targeting .NET Standard 2.1.

I would request if it is possible to keep the targeting .NET Standard 2.0?
This would keep it compatible with .NET Framework which I am sure a lot of people will want to still use (which is something Angus mentioned he wanted to ensure in #12).

I see that as of #593 the only difference is a missing Combine hash function. A collaborator kindly provided me with this custom implementation. While this works in two of my projects using clipper2, I am not familiar with the original function in .NET Standard 2.1 so I can't guarantee that this custom implementation will accurately mimic the original Combine function. Maybe someone that is more familiar with this can help/confirm?

using System;
using System.Collections.Generic;
using System.Text;

namespace Clipper2Lib
{
  public static class HashCode
  {
    public static int Combine<T1, T2>(T1 value1, T2 value2)
    {
      unchecked
      {
        int hash1 = value1 != null ? value1.GetHashCode() : 0;
        int hash2 = value2 != null ? value2.GetHashCode() : 0;

        int hash = 17;
        hash = hash * 23 + hash1;
        hash = hash * 23 + hash2;

        return hash;
      }
    }
  }
}

```

[AngusJohnson]

Thanks for the feedback.
Honestly I have very limited experience with C# and I'm still unsure how best to package Clipper2 in C#.
Nevertheless, I seem to recall that there are either significant language improvements (moving from .NET Standard 2.0 to 2.1) and/or compatability issues with when compiling with the support (.NET 6.0) sample applications.
Anyhow, I'm certainly open to suggestions on how best to do this.