arm64 opt in - NU1004 The project's runtime identifiers have changed - ".NETFramework,Version=v4.8/win-arm64"

This is a request to make the `win-arm64` RID opt in for non-sdk style projects, because currently it creates a flag day for `package.lock.json` - old projects won't build on new build tools and (perhaps) new ones won't build on old build tools.

If you're adding arm64 support to an existing project, I don't think it's unreasonable to add something to the project file.

introduced in: https://github.com/dotnet/NuGet.BuildTasks/pull/156
originally reported: https://github.com/NuGet/Home/issues/12711