Update dependencies from https://github.com/dotnet/standard build 20190912.5 (#41073)

- NETStandard.Library - 2.1.0

Author: dotnet-maestro[bot]

NuGet.config

@@ -6,6 +6,9 @@
   </fallbackPackageFolders>
   <packageSources>
     <clear />
+    <!--Begin: Package sources managed by Dependency Flow automation. Do not edit the sources below.-->
+    <add key="darc-pub-dotnet-standard-a5b5f2e" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/darc-pub-dotnet-standard-a5b5f2e1/nuget/v3/index.json" />
+    <!--End: Package sources managed by Dependency Flow automation. Do not edit the sources above.-->
     <add key="dotnet-core" value="https://dotnetfeed.blob.core.windows.net/dotnet-core/index.json" />
     <add key="dotnet-coreclr" value="https://dotnetfeed.blob.core.windows.net/dotnet-coreclr/index.json" />
     <add key="nuget.org" value="https://api.nuget.org/v3/index.json" />

eng/Version.Details.xml

@@ -38,9 +38,9 @@
       <Uri>https://github.com/dotnet/arcade</Uri>
       <Sha>8eb29ba860a3cfcfe68f9a8256caa7efc1f1aaba</Sha>
     </Dependency>
-    <Dependency Name="NETStandard.Library" Version="2.1.0-prerelease.19462.4">
+    <Dependency Name="NETStandard.Library" Version="2.1.0">
       <Uri>https://github.com/dotnet/standard</Uri>
-      <Sha>9dda25e8818255879640d0a8b63c5de8fb724820</Sha>
+      <Sha>a5b5f2e1e369972c8ff1e2183979fab6099f52ef</Sha>
     </Dependency>
     <Dependency Name="Microsoft.DotNet.Helix.Sdk" Version="2.0.0-beta.19461.7">
       <Uri>https://github.com/dotnet/arcade</Uri>

eng/Versions.props

@@ -52,7 +52,7 @@
     <MicrosoftNETCorePlatformsPackageVersion>3.0.0-preview9.19409.15</MicrosoftNETCorePlatformsPackageVersion>
     <runtimenativeSystemIOPortsPackageVersion>4.6.0-preview9.19409.15</runtimenativeSystemIOPortsPackageVersion>
     <!-- Standard dependencies -->
-    <NETStandardLibraryPackageVersion>2.1.0-prerelease.19462.4</NETStandardLibraryPackageVersion>
+    <NETStandardLibraryPackageVersion>2.1.0</NETStandardLibraryPackageVersion>
     <!-- dotnet-optimization dependencies -->
     <optimizationwindows_ntx64IBCCoreFxPackageVersion>99.99.99-master-20190905.1</optimizationwindows_ntx64IBCCoreFxPackageVersion>
     <!-- sni -->