Downgrade TFMs to NetMinimum and NetFrameworkMinimum

[ViktorHofer]

In an effort to decouple Arcade from the latest .NET SDK, downgrade project TFMs to the minimum in support versions. This means that projects now consistently target net8.0 when not building source-only.

Also set NetFrameworkMinimum to net472 as the Arcade tools and packages don't need to support a lower version and that simplifies conditions and package dependencies.

To double check:

• [ ] The right tests are in and the right validation has happened. Guidance: https://github.com/dotnet/arcade/blob/main/Documentation/Validation.md