[MAINT]: Upgrade to .NET 9.0

[IEvangelist]

Describe the need

We should upgrade to .NET 9.0, now that it released a few days ago. Also, as part of this there are several things that can be cleaned up as part of this effort.

SDK Version

No

API Version

No

Code of Conduct

• [X] I agree to follow this project's Code of Conduct

[github-actions[bot]]

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[NatMarchand]

Hi! Why would you consider such an upgrade? Dotnet8 is the current LTS framework and upgrading to net9 without net8 support means that a lot of us cannot upgrade. Remember, net9 support ends BEFORE net8 support. I'm curious to known what do you need in net9 that is not already included in net8/csharp12. From your current PR, it seems that nothing is required.