Adding 2.1 feed for System.Drawing fix

Author: joperezr

NuGet.config

@@ -16,6 +16,8 @@
     <add key="dotnet-eng" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-eng/nuget/v3/index.json" />
     <add key="dotnet5" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet5/nuget/v3/index.json" />
     <add key="dotnet5-transport" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet5-transport/nuget/v3/index.json" />
+    <!-- Harvesting feed from 2.1 -->
+    <add key="darc-int-dotnet-corefx-43e382ec" value="https://pkgs.dev.azure.com/dnceng/internal/_packaging/darc-int-dotnet-corefx-43e382ec/nuget/v3/index.json" />
   </packageSources>
   <disabledPackageSources>
     <clear />