Add support for .NET 9

[sailro]

Description

.NET 9 is now GA, so add support for it:

• added net9.0 to target frameworks of the packable project (keeping older versions as well, so it should not break existing users)
• bumped test code projects to net9.0
• adjusted build and publish workflows to use net9.0

Remarks:

• The target framework 'net7.0' is now out of support and will not receive security updates in the future. You'll hit a NETSDK1138 warning if you keep this one.
• I think it would also be a good idea to upgrade the dependencies (in particular to fix known vulnerabilities). I can do that in a new PR if you want.
• Even on master I'm having 40 tests passing and 13 failing with a NRE (exact same thing with this branch). Issue106, Issue99, CheckVersion, IntegrationTest and IntegrationTestCopyFolder. Do I need to setup something first?

Thanks!

Type of change

• New feature (non-breaking change which adds functionality)

Checklist

• I have performed a self-review of my code
• I did test corresponding changes on Windows

[what-the-diff]

PR Summary

• Updated .NET Version in Workflow Files
  The version of .NET used in the GitHub Actions workflow files has been upgraded from .NET 8 to .NET 9. This applies to both the build.yml and publish.yml files. This change helps ensure our workflows are running on the latest version of .NET, increasing compatibility and accessing new features.

• Changed dotnet-version in Workflow Files
  The specific version of dotnet used in the build.yml and publish.yml files has been updated from 8.0.x to 9.0.x. This means that our build and publish processes will use the latest .NET version, leading to increased efficiency and compatibility.

• Expanded Target Frameworks in Husky.csproj
  The target frameworks of Husky.csproj have been expanded to include .NET 9.0, allowing the code to be built against multiple versions of the .NET framework.

• Updated Target Framework in Test Project Files
  The target framework of HuskyIntegrationTests.csproj and HuskyTest.csproj has been updated from .NET 8.0 to .NET 9.0. This change ensures that our tests run on the same version as our actual application, ensuring coherent and reliable testing.

[alirezanet]

LGTM 👌Awesome thanks, sorry for the late review 💐