Get rid of custom reference resolution system, use built-in nuget features

The custom logic at https://github.com/dotnet/aspnetcore/blob/main/eng/targets/ResolveReferences.targets (and similar) was built a while ago, in order to solve issues like central dependency management & keeping references consistent in servicing. Those problems are now either solved by built-in Nuget features like [Central Package Management](https://learn.microsoft.com/en-us/nuget/consume-packages/central-package-management) and transitive pinning, or mitigated by the fact that we no longer do incremental servicing. Plus we have better validation for breaking reference changes now.

We should get rid of our custom stuff & use the built-in nuget features so we can get on the happy path - this will help future-proof our infra, and likely make the build faster/more reliable too.