Support .NET 9

[PKoning]

My project contains a build target that runs the pum-gen command after build. Locally, I can build my project and the generator runs fine. However, I also build my project in a GitLab pipeline, in which I'm using the .NET 9 SDK Docker image (mcr.microsoft.com/dotnet/sdk:9.0). I get errors, because the generator tool can only run on .NET 8.

So, I'd the generator tool to support running on .NET 9 as well.

FYI, the error message I get in my pipeline:

You must install or update .NET to run this application.
  
  App: /root/.dotnet/tools/puml-gen
  Architecture: x64
  Framework: 'Microsoft.NETCore.App', version '8.0.0' (x64)
  .NET location: /usr/share/dotnet
  
  The following frameworks were found:
    9.0.3 at [/usr/share/dotnet/shared/Microsoft.NETCore.App]

[pkoningmypoiworld]

@pierre3 Can we get any update on this? Or should we consider this project as EOL? (Which is totally fine, but it would be nice to know)