Fix GNOME Shell maximum version

to match metadata.json

gnome-shell-extension-flypie 26-1 (which was uploaded to Unstable earlier today) is uninstallable.

When metadata.json lists 46 as the maximum version supported, please use (<< 47~) in debian/control to show that it is compatible with any 46.* version but not with 47 or any of the 47 pre-releases. (The Debian GNOME team usually does package pre-releases of GNOME Shell at least in Experimental.)

If you remember, you can also use urgency=high in your upload since many of the other parts of this transition will be ready for migration to Testing earlier than 5 days.

cc/ @lumin