ReferenceAssemblies for .NET 9 are p1

[333fred]

See context from https://github.com/dotnet/roslyn/discussions/77207#discussioncomment-12198009. The gist is that 1.1.2 misleadingly ships with a ".NET 9" set of references, but those references are actually p1, and are missing most of the interesting changes in .NET 9. We should update that reference to the RTM version, and further never ship a pre-release version of the .NET reference assemblies without clearly labeling them as such in this library.