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

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](https://github.com/getsentry/sentry-dotnet/blob/cc1b3e822c0477d2e39b35e4ae552a263030e0b5/Directory.Build.props#L29).

_Originally posted by @jamescrosswell in https://github.com/getsentry/sentry-dotnet/pull/5529#discussion_r3975401122_

Start with Directory.Build.props at the linked target-framework setting, then read the ## Upgrading to a new .NET major version section in CONTRIBUTING.md. Trace how the device tests currently choose their TFM. Done means the tests use the latest TFM from the build props without the annual manual update.

Written by the indexing model from the issue text.