chore: Add support for .NET 10 to tests

[CurtHagenlocher]

What's Changed

Add support for .NET 10 to tests and examples
Remove support for .NET 6

[CurtHagenlocher]

Probably need to update the docker images?

EDIT: Ah, the SDK is being installed by the GitHub setup-dotnet action which does not yet have support for .NET 10. Rather than do something more complicated now, I think we'll wait until either that changes or it's announced that it won't.

[adamreeve]

I don't think the setup-dotnet action is the problem, it looks like .NET 10 SDK is actually installed on the runner images already, eg. https://github.com/apache/arrow-dotnet/actions/runs/19713256024/job/56479218949#step:3:12

For the integration tests, I think the Docker image in the main arrow repository needs updating if I've understood how these work: https://github.com/apache/arrow/blob/fa41b2674bfab627ec1c69b4b4fb52d9caff8c15/ci/docker/ubuntu-22.04-csharp.dockerfile#L19

And it looks like the problem with the verification step is that the tests are run with the .NET 8 SDK as well as .NET 10, and when run with .NET 8 they can't build the .NET 10 target. We might just need to pass the --framework argument to dotnet test and only run the tests for one framework at a time?

[CurtHagenlocher]

Yeah, now that you mention it I can see that the "Setup .NET" step does indeed appear to be trying to install .NET 10. And it's interesting that this seems working on Ubuntu but not on Windows or MacOS.