ObserveOnDispatcher is not available in a .NET 5 WPF app

[twtdev]

Library/environment:
System.Reactive 5.0.0
.NET 5 (WPF)
Windows version 10.0.19042
Visual Studio version 16.8.3

What is the use case or problem?
Create a brand new project using the WPF App (.NET) template, change the target framework to .NET 5.0 in the project's properties page, add the System.Reactive 5.0.0 nuget package. Then in MainWindow.xaml.cs, add the following code to MainWindow():

IObservable<string> singleValue = Observable.Return<string>("Value");
singleValue.ObserveOnDispatcher();

What is the expected outcome?
That there would be no errors.

What is the actual outcome?
A compile-time error: IObservable does not contain a definition for 'ObserveOnDispatcher'...

Do you have a code snippet or project that reproduces the problem?
Yes, can be provided if required, but probably faster just to try the steps described above

Also, my .csproj file contains the following:

<PropertyGroup>
  <OutputType>WinExe</OutputType>
  <TargetFramework>net5.0-windows</TargetFramework>
  <UseWPF>true</UseWPF>
</PropertyGroup>

[twtdev]

Solved: I changed the TargetFramework value to net5.0-windows10.0.19041