Update ReactiveUI and Rx base version (#16719)

Co-authored-by: Julien Lebosquain <[email protected]>

build/ReactiveUI.props

@@ -1,5 +1,5 @@
 <Project DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
   <ItemGroup>
-    <PackageReference Include="ReactiveUI" Version="18.3.1" />
+    <PackageReference Include="ReactiveUI" Version="20.1.1" />
   </ItemGroup>
 </Project>

build/Rx.props

@@ -1,5 +1,5 @@
 <Project DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
   <ItemGroup>
-    <PackageReference Include="System.Reactive" Version="5.0" />
+    <PackageReference Include="System.Reactive" Version="6.0.1" />
   </ItemGroup>
 </Project>