dconf is a database that is used as a semi-dumb storage backend – it will not contain values for settings you have not changed. You should use gsettings command, which actually ensures correct values are written according to GSettings schmas: gsettings set org.gnome.shell disable-extension-version-validation true
Just note that the version validation is there for a reason and extensions for older versions of GNOME Shell might not work or using them might cause shell instability or other issues.