Allow building with webkitgtk 4.1
4.1 is the same as 4.0 except that it uses libsoup3 instead of libsoup2.4
GNOME 43 uses libsoup3 and webkitgtk 4.1 or 5.0 by default. (5.0 is the GTK4 version; 4.1 is for GTK3). But older distros don't have webkitgtk 4.1 so we probably still want to support 4.0 if it's not too inconvenient.