Use appstream-glib implementation of content rating systems
Move the content rating system implementation into appstream-glib so it can be shared by appstream-glib, gnome-software and libmalcontent-ui.
This doesn’t bump the gnome-software dependency on appstream-glib as only 0.7.14 is available in Debian Stable, which is used for CI. Instead, it adds a load of compatibility
#defines. In time, once the hard dependency is bumped,
gs-content-rating.[ch] can be dropped entirely.