Update xdt version

The xdt version we are building w/source-build (5.0.0-preview.21431.1) is older and should be upgraded in order to remove this [patch](https://github.com/dotnet/installer/blob/main/src/SourceBuild/tarball/patches/xdt/0001-Also-build-for-netstandard2.0-to-support-NuGet.patch) was backported at the beginning of 7.0 but has never flowed in.