Add dotnet 9 build target

[BaptisteV]

Hi!

I'd like to contribute. Are you interested in adding dotnet 9 as a build target ?
Also, are you interested in dropping .net framework and .net standard support ?
The main benefits would be :
Wider async support for IO operations (using File.WriteAllBytesAsync and File.ReadAllLinesAsync for exemple)
Recent c# langage features like file scoped namespaces.