Attempt feed cleanup (#20395)

* Attempt feed cleanup

Prepping for repos no longer publishing to dotnet-core

* Add 3.1 feeds

NuGet.config

@@ -2,15 +2,11 @@
 <configuration>
   <packageSources>
     <clear />
-    <!--Begin: Package sources managed by Dependency Flow automation. Do not edit the sources below.-->
-    <add key="darc-pub-dotnet-core-setup-916b5cb" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/darc-pub-dotnet-core-setup-916b5cba/nuget/v3/index.json" />
-    <add key="darc-pub-aspnet-Extensions-1286a6f" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/darc-pub-aspnet-Extensions-1286a6ff/nuget/v3/index.json" />
-    <!--End: Package sources managed by Dependency Flow automation. Do not edit the sources above.-->
-    <add key="aspnet-extensions" value="https://dotnetfeed.blob.core.windows.net/aspnet-extensions/index.json" />
-    <add key="dotnet-core" value="https://dotnetfeed.blob.core.windows.net/dotnet-core/index.json" />
     <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" />
+    <add key="dotnet3.1" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet3.1/nuget/v3/index.json" />
+    <add key="dotnet3.1-transport" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet3.1-transport/nuget/v3/index.json" />
     <add key="roslyn" value="https://dotnet.myget.org/F/roslyn/api/v3/index.json" />
     <add key="nuget.org" value="https://api.nuget.org/v3/index.json" />
   </packageSources>