update System.Reactive to .net5

build/Rx.props

@@ -1,5 +1,5 @@
 <Project DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
   <ItemGroup>
-    <PackageReference Include="System.Reactive" Version="4.4.1" />
+    <PackageReference Include="System.Reactive" Version="5.0" />
   </ItemGroup>
 </Project>