net6 no longer supported by examples

[nohwnd]

Describe the bug

17.14.0 no longer supports net6.0 tfms, it would be better to restore it back to 17.13.0 in samples/public/Directory.Build.props.

[Youssef1313]

Do we have any samples on net6.0 or net7.0? Why not update them to net8.0 instead of downgrading VSTest?

[nohwnd]

Just because we probably will have to keep the version referenced from MTP to stay 17.13.0 until we drop net6 (and other unsupported frameworks) from it to avoid breaking change.

Or drop the support in mtp and mstest.

[Youssef1313]

Makes sense. I think as we dropped support for those in VSTest, we can hopefully do the same for MTP and MSTest, maybe in 3.10?