Update NuGet.config

Add the dotnet8 feeds back

Author: mmitche

NuGet.config

@@ -10,6 +10,8 @@
     <add key="dotnet-eng" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-eng/nuget/v3/index.json" />
     <add key="dotnet-libraries" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-libraries/nuget/v3/index.json" />
     <add key="vs-buildservices" value="https://pkgs.dev.azure.com/azure-public/vside/_packaging/vs-buildservices/nuget/v3/index.json" />
+    <add key="dotnet8" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet8/nuget/v3/index.json" />
+    <add key="dotnet8-transport" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet8-transport/nuget/v3/index.json" />
     <add key="dotnet9" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet9/nuget/v3/index.json" />
     <add key="dotnet9-transport" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet9-transport/nuget/v3/index.json" />
   </packageSources>