Revert "Revert nuget.config changes"

This reverts commit 0075cd3.

Author: hez2010

nuget.config

@@ -1,6 +1,8 @@
 <?xml version="1.0" encoding="utf-8"?>
 <configuration>
     <packageSources>
+        <clear />
+        <add key="NuGet Gallery" value="https://api.nuget.org/v3/index.json" />
         <add key="Project Packages" value="src/Files.App/Assets/Libraries/" />
         <add key="Toolkit Labs" value="https://pkgs.dev.azure.com/dotnet/CommunityToolkit/_packaging/CommunityToolkit-Labs/nuget/v3/index.json" />
     </packageSources>