Package System.Reactive.Windows.Threading 5.0.0 is not compatible with net462.

#### Bug

Hello,

I have a project that targets 4.6.2 and is written in VB.Net and using WPF.
When trying to update RX to the latest version the package System.Reactive.Windows.Threading fails update with error below.

All other Rx packages like System.Reactive itself update without problem.
It is also strange that WinForms app that targets 4.6.2 seems be able to update to the same version.

> Which library version?

System.Reactive.Windows.Threading 5.0.0

> What are the platform(s), environment(s) and related component version(s)?

WPF, .Net 4.6.2

> What is the use case or problem?

Nuget package doesn't target 4.6.2. And fails to install/update.

> What is the expected outcome?

Package should install.

> What is the actual outcome?

Severity Code Description Project File Line Suppression State
Error NU1202 Package System.Reactive.Windows.Threading 5.0.0 is not compatible with net462 (.NETFramework,Version=v4.6.2). Package System.Reactive.Windows.Threading 5.0.0 supports:
- net472 (.NETFramework,Version=v4.7.2)
- uap10.0.16299 (UAP,Version=v10.0.16299) WpfApp1

> What is the stacktrace of the exception(s) if any?

None.

> Do you have a code snippet or project that reproduces the problem?

Problem reproduces with a blank VisualBasic WPF app that targets 4.6.2.