According to this post: .NET 5 for Rhino 8? - #2 by stevebaer
framework for Rhino 7 is, and most probably for Rhino 8 will be .NET Framework 4.8. There might be transition to .NET 6, but .NET Framework 4.8 should also be supported in some manner.
If you choose C# and Visual Studio there are plugin templates for Rhino 7: