Note: PR #5154 is upgrading all target frameworks to net10.0-*, which will fundamentally solve this issue.

This PR provides an immediate fix by enforcing .NET 9 SDK until #5154 is merged.

However, I believe global.json is still valuable for ensuring consistent SDK versions across all development environments. Once #5154 is merged, the SDK version in global.json should be updated to 10.0.100.

What are the semantics of this? In particular, would it prevent building things on previous versions? We've been following the EOL dates of .NET, and if we continue to do that we would have to support NET 8 for another year (and in practice we rarely actively drop support unless it causes issues) and we have to support .NET Framework versions basically until Microsoft says otherwise (the length of .NET Framework support requirements is probably the only thing that would make us reevaluate this choice).

@bclehmann

Thanks for the question! To clarify, ScottPlot already cannot be built with the .NET 8 SDK. Because the project targets net9.0, .NET 8 SDK immediately fails with:

NETSDK1045: The current .NET SDK does not support targeting .NET 9.0.

So this PR does not introduce any new restriction. It simply makes the existing requirement explicit and prevents build breaks on .NET 10+ SDKs.

Regarding EOL policy, targeting net8.0 doesn't require using the .NET 8 SDK — the .NET 9 SDK fully supports building net8.0 projects.

I hope this clarifies the intent of the PR!

To further clarify, the SDK is simply the build toolchain — the .NET 9 SDK can build net8.0, net462, and netstandard2.0 targets without any issues.

Also, if ScottPlot currently builds fine on your machine, that confirms you already have the .NET 9 SDK installed.