Update package source key in NuGet.config

Author: Shyam-Gupta

NuGet.config

@@ -21,7 +21,7 @@
   </packageSources>
   <packageSourceMapping>
     <clear />
-    <packageSource key="darc-pub-dotnet-dotnet-4ef9fba">
+    <packageSource key="darc-pub-dotnet-dotnet-976695d">
       <package pattern="Microsoft.*" />
       <package pattern="runtime.*" />
       <package pattern="System.*" />