Here's the PR for #40 which is the basic upgrade to .NET 9.

Two notes:

1. I've changed the sample Dockerfile to use the basic base images of .NET to not having to think about if there's a "bullseye" image for .NET 9.
2. The project is building but with an obsoletion warning: "Loading certificate data through the constructor or Import is obsolete." which at some point should be resolved by adopting SYSLIB0057: X509Certificate2 and X509Certificate constructors for binary and file content are obsolete. For me personally this is out of scope at this moment in time and I think someone can tackle this at a later stage.