Remove links to dead developer-old.gnome.org
See https://discourse.gnome.org/t/retirement-of-developer-old-gnome-org/20991
./GNOME/devdocsgjs/lib/docs/scrapers/gtk.rb: self.base_url = "https://developer-old.gnome.org/gtk4/#{self.version}/"
https://developer-old.gnome.org/gtk4/
is dead. Should probably use https://docs.gtk.org/gtk4/ instead.
Also, all https://developer.gnome.org/gtk3/
links should likely be replaced with https://docs.gtk.org/gtk3/