Automatically update the latest TFM in device tests from Directory.Build.props #5565

We want to run the tests on the latest TFM. We change this once per year.

There's a blurb now in the ## Upgrading to a new .NET major version section of the CONTRIBUTING.md explaining what we do.

We could automate this further by reading it directly from the build props.

Originally posted by @jamescrosswell in #5529 (comment)