Clean up/simplify target frameworks

[Miista]

Ideally, it would be enough for the Pose project itself to target just the lowest possible target framework—only specifying a specific target framework when absolutely necessary (for instance, when a specific target framework requires special handling).

The tests, on the other hand, should target all possible target frameworks.

I can't speak to the feasibility of this. I'm merely looking to clean it up (if possible).