Migrate aspnetcore 2.3 code to dotnet/maintenance-packages

To be done after https://github.com/dotnet/aspnetcore/issues/49850. Move all supported libraries to https://github.com/dotnet/maintenance-packages, which will be the long-term home for 2.1 libraries. We'll still need to figure out what to do about the installers built out of aspnetcore 2.1, which have special logic that isn't already accounted for in `maintenance-packages`.