Sdk tooling target framework should be net8.0

[mmitche]

SDK tooling TFM is currently net7.0. This should be updated to net8.0.

While the SDK does use a property for this in Directory.Build.Props, it does not appear to be used in all places that should (e.g. test assets).

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[marcpopMSFT]

The SDK is now targeting net8.0 other than the resolver and our tests. Is that what you had in mind for this issue?
#28563

Active one for tests:
#29316