Work around incompatibility with System.ValueTuple 4.6.1 and add .NET 10 to test matrix

[jnm2]

Fixes #34. This workaround is one of the three options identified at https://github.com/buvinghausen/TaskTupleAwaiter/issues/34#issuecomment-3519340657. I prefer this approach because it targets the minimum API set that the package needs. Adding an additional newer target, while keeping the older one, is only useful if it's consuming an API that's not available in the older target. (Like we do with .NET 8 and its ConfigureAwaitOptions.)