Add target that validates that every packable project contains a $(NetCurrent) target framework

_`$(NetCurrent)` refers to the TFM of the major release of .NET that the Arcade SDK and the toolset SDK aligns with._

Source building .NET is about generating assets for the current version of the product. At the time of writing this refers to .NET 8 and the net8.0 TFM. While the source-build team is actively working on removing .NET Framework nodes from the build and pre-build graph, .NET Standard and out-of-support .NETCoreApp versions are still dependent on.

We should consider adding a validation target into Arcade that verifies that every packable project targets at least `$(NetCurrent)`.

The reasons for doing so contribute to source build's sustainability:
1. Reduces size significantly by avoiding .NETStandard targeting packs altogether, and .NETCoreApp targeting packs for out-of-support runtimes.
2. Eliminates SBRP prebuilds when the package in question is still live produced in the build graph.
3. Makes it easier to rev to the next major version of .NET by avoiding new prebuilds
4. Faster source builds because of less nodes in the build graph.

While some repositories like dotnet/runtime already offer `$(NetCurrent)` assets for all packages that are part of source-build, not every repository does that. We could manually roll that enforcement out and while doing that also making sure that the floating TFM property is used `$(NetCurrent)` so that repositories automatically rev their assets to the next major version of .NET.

cc @MichaelSimons @mmitche @ericstj