AppCriteria.md: Update URL to GNOME Code of Conduct
The GNOME Code of Conduct web page that used to be located at https://wiki.gnome.org/Foundation/CodeOfConduct has been moved to https://conduct.gnome.org/.
The GNOME Code of Conduct web page that used to be located at https://wiki.gnome.org/Foundation/CodeOfConduct has been moved to https://conduct.gnome.org/.