Fix CI build by using latest Windows SDK in C# project (#1516)

* Attempt "latest SDK" setting

* Update TargetPlatformVersion to use WindowsSDKVersion

* Fix TargetPlatformVersion trimming in project file

Author: DefaultRyan

test/nuget/TestRuntimeComponentCSharp/TestRuntimeComponentCSharp.csproj

@@ -11,7 +11,7 @@
     <AssemblyName>TestRuntimeComponentCSharp</AssemblyName>
     <DefaultLanguage>en-US</DefaultLanguage>
     <TargetPlatformIdentifier>UAP</TargetPlatformIdentifier>
-    <TargetPlatformVersion Condition=" '$(TargetPlatformVersion)' == '' ">10.0.22621.0</TargetPlatformVersion>
+    <TargetPlatformVersion Condition=" '$(TargetPlatformVersion)' == '' ">$(WindowsSDKVersion.TrimEnd('\'))</TargetPlatformVersion>
     <TargetPlatformMinVersion>10.0.18362.0</TargetPlatformMinVersion>
     <MinimumVisualStudioVersion>14</MinimumVisualStudioVersion>
     <FileAlignment>512</FileAlignment>
@@ -93,4 +93,4 @@
   <Target Name="AfterBuild">
   </Target>
   -->
-</Project>
+</Project>