[4.8.x] subprojects: Pin GLib to version 2.74.x
GLib 2.74.x and GTK 4.8.x both correspond to the GNOME 43 cycle. Keeping GLib on the 2.74.x branch avoids a CI regression when GLib git main depends on a newer Meson version than the one GTK's CI installs, which has happened in 2.75.x as a result of glib!3077 (merged).