Upgrade to .NET 9

[BNAndras]

I borrowed the updated package versions from the C# test runner. The NuGet audit flag was needed since no internet access means the audit couldn't access NuGet's API.

[github-actions]

Hello 👋 Thanks for your PR.

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[BNAndras]

Before this gets merged in, I should update the track exercises and merge everything together. I'm expecting it should be pretty straighforward.

[ErikSchierboom]

The NugetAudit thing is weird. I can look into fixing this.

[ErikSchierboom]

@BNAndras I've fixed the remaining issues. Let me know when you want to merge.

[BNAndras]

Thanks! I just need to sync the update PR vbproj files in the main track repo, and then I can merge in everything later today.