Add .NET 9 as a supported framework

[patrikwlund]

What are you trying to accomplish?

I want to add .NET 9 to the list of supported frameworks, and thus to the "compatibility matrix", so that updates of packages targeting older versions of the framework are allowed.

Package update are currently rejected with the following message when not directly targeting NET 9:

The package <package-name>.<package-version> is not compatible. Incompatible project frameworks: net9.0

Fixes: #11516

Anything you want to highlight for special attention from reviewers?

How will you know you've accomplished your goal?

.NET 9 projects receive updates for packages that target older (pre 9) framework versions.

Checklist

• I have run the complete test suite to ensure all tests and linters pass.
• I have thoroughly tested my code changes to ensure they work as expected, including adding additional tests for new functionality.
• I have written clear and descriptive commit messages.
• I have provided a detailed description of the changes in the pull request, including the problem it addresses, how it fixes the problem, and any relevant details about the implementation.
• I have ensured that the code is well-documented and easy to understand.

[martincostello]

In a separate PR, would be good to have this refactored in a way that unless/until something comes along that needs specific attention because it breaks current assumptions/patterns, this "just works" for future .NET versions so it doesn't need changing again for .NET 10, 11 etc. for the form net{major.minor}.

[patrikwlund]

would be good to have this refactored

I agree. Making these additions felt a bit silly.

[patrikwlund]

It looks like one of the smoke tests failed because of a 504 when fetching https://api.nuget.org/v3-flatcontainer/system.buffers/index.json. Seems like a temporary network issue. Probably have to re-run the failed tests.