Some of the .ui files already use features
requiring newer versions than 3.1.13,
e.g. glade 3.40.0 gives this warning when saving
plugins/interface_view.ui:

> [iface_view_frame > alignment1 > vbox_ifaces > expander_value > alignment24 > hbox5 > table1 > label34]
> 	Property 'X align' of object class 'Label' was introduced in gtk+ 3.16

Since Gtk 3.24 was released about 5 years ago and
requiring it probably also simplifies migration
to Gtk 4, use that as new minimum version.