Microsoft.Windows.SDK.NET.Ref package not referenced when targeting .NET 5.0 in .NET 9.0 RC.2 SDK

### Description
A simple project targeting e.g. `net5.0-windows10.0.17763.0` will fail to resolve UWP types like `Windows.Devices.Power.Battery` because `Microsoft.Windows.SDK.NET.Ref` references are not passed to C# compiler.

```
error CS0246: The type or namespace name 'Windows' could not be found (are you missing a using directive or an assembly reference?)
error CS0246: The type or namespace name 'Battery' could not be found (are you missing a using directive or an assembly reference?)
```

This was working in RC.1.

Changing the target framework version to `net6.0-windows10.0.17763.0` resolves this problem.

.NET 5.0 is not supported, but is this intentional?

### To Reproduce
1. Create any .NET project targeting `net5.0-windows10.0.17763.0` or any other version of Win SDK as long as the target framework is `net5.0`.
2. Reference any UWP type like `Windows.Devices.Power.Battery`.
3. `dotnet build` with .NET 9.0 RC2 (I tried with the 6th November nightly build and the behavior was the same).