filechooser preferences: update g_settings schema path for compatibility with gtk4

this solves a bug where nautilus would crash immediately on start if the system lacks a gtk3 installation:

$ nautilus
(org.gnome.Nautilus): GLib-GIO-ERROR **: Settings schema 'org.gtk.Settings.FileChooser' is not installed

gtk renamed this settings schema here:

• gtk@5c31c721

now that nautilus depends specifically on gtk4, it needs to use that gtk4 settings schemas.