Remove dependency on Newtonsoft.Json

Now that we have https://www.nuget.org/packages/System.Text.Json/, we should remove our dependency on Newtonsoft.Json, at a minimum in our core Microsoft.ML nuget package.

This has the following advantages:

1. Less assemblies to carry around/deploy on .NET Core, since `System.Text.Json` is part of the shared framework.
2. It frees up the calling program to pick which version of Newtonsoft.Json it can use, instead of Microsoft.ML saying that it must use one `10.0.3` or higher.
3. `System.Text.Json` is faster (although we don't really use JSON code on any critical path).
4. In the core Microsoft.ML nuget package, this is the only 3rd party dependency. So removing it means ML.NET only depends on `System` packages.