Update to latest extras

Rx.NET/Source/Directory.build.props

@@ -22,7 +22,6 @@
     <EmbedUntrackedSources>true</EmbedUntrackedSources>
     <PublishRepositoryUrl>true</PublishRepositoryUrl>
     <LangVersion>latest</LangVersion>
-    <ExtrasUwpMetaPackageVersion>6.1.9</ExtrasUwpMetaPackageVersion>
   </PropertyGroup>
 
 </Project>

Rx.NET/Source/global.json

@@ -3,6 +3,6 @@
     "version": "3.0.100-preview"
   },
   "msbuild-sdks": {
-    "MSBuild.Sdk.Extras": "2.0.0-preview.14"
+    "MSBuild.Sdk.Extras": "2.0.0-preview.21"
   }
 }

azure-pipelines.rx.yml

@@ -23,7 +23,7 @@ pr:
 jobs:
 - job: Build
   pool:
-    vmImage: vs2017-win2016
+    vmImage: windows-2019
 
   variables: 
     BuildConfiguration: Release