Bump System.Threading.Tasks.Extensions in /Rx.NET/Source

Bumps [System.Threading.Tasks.Extensions](https://github.com/dotnet/corefx) from 4.5.3 to 4.5.4.
- [Release notes](https://github.com/dotnet/corefx/releases)
- [Commits](https://github.com/dotnet/corefx/commits)

Signed-off-by: dependabot-preview[bot] <[email protected]>

Rx.NET/Source/Directory.build.targets

@@ -26,7 +26,7 @@
 
 
   <ItemGroup Condition="'$(TargetFramework)' != 'netcoreapp3.0'">
-    <PackageReference Include="System.Threading.Tasks.Extensions" Version="4.5.3" />
+    <PackageReference Include="System.Threading.Tasks.Extensions" Version="4.5.4" />
   </ItemGroup>
 
   <Target Name="AddCommitHashToAssemblyAttributes" BeforeTargets="GetAssemblyAttributes">