Merge pull request #961 from dotnet/dependabot/nuget/Rx.NET/Source/System.Threading.Tasks.Extensions-4.5.3

Bump System.Threading.Tasks.Extensions from 4.5.2 to 4.5.3 in /Rx.NET/Source

Rx.NET/Source/Directory.build.targets

@@ -26,7 +26,7 @@
 
 
   <ItemGroup Condition="'$(TargetFramework)' != 'netcoreapp3.0'">
-    <PackageReference Include="System.Threading.Tasks.Extensions" Version="4.5.2" />
+    <PackageReference Include="System.Threading.Tasks.Extensions" Version="4.5.3" />
   </ItemGroup>
 
   <Target Name="AddCommitHashToAssemblyAttributes" BeforeTargets="GetAssemblyAttributes">