Fixup feeds (#615)

Co-authored-by: Matt Mitchell <[email protected]>

Author: dotnet-bot

nuget.config

@@ -2,7 +2,7 @@
 <configuration>
   <packageSources>
     <clear />
-    <add key="nuget.org" value="https://api.nuget.org/v3/index.json" />
+    <add key="dotnet-public" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-public/nuget/v3/index.json" />
     <add key="dotnet-myget-legacy" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/myget-legacy/nuget/v3/index.json" />
     <add key="VSIDE vs-impl" value="https://pkgs.dev.azure.com/azure-public/vside/_packaging/vs-impl/nuget/v3/index.json" />
   </packageSources>