App Config’s root element should be namespace-less

The root element for Application config file is <configuration>

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
</configuration>

The application config file is shared with CLR and Windows SxS.

SxS dictates that the root element is <configuration>, and it should be without namespace. If the application config file does not follow the rule, SxS will refuse to start the process.