Add missing package mapping for net9 feed (#2366)

Author: JamesNK

nuget.config

@@ -23,6 +23,9 @@
     <packageSource key="dotnet8">
       <package pattern="*" />
     </packageSource>
+    <packageSource key="dotnet9">
+      <package pattern="*" />
+    </packageSource>
     <packageSource key="grpc">
       <package pattern="*" />
     </packageSource>