build: Bump requirements for Xfce 4.18

There are some deprecations warnings that must be fixed here (see xfce4-dev-tools!35 (comment 60848)), like

xfce-titled-dialog.c:112:3: warning: 'g_type_class_add_private' is deprecated [-Wdeprecated-declarations]
  112 |   g_type_class_add_private (klass, sizeof (XfceTitledDialogPrivate));
      |   ^~~~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/include/glib-2.0/gobject/gobject.h:26,
                 from /usr/include/glib-2.0/gobject/gbinding.h:31,
                 from /usr/include/glib-2.0/glib-object.h:24,
                 from /opt/include/xfce4/libxfce4util/xfce-kiosk.h:30,
                 from /opt/include/xfce4/libxfce4util/libxfce4util.h:33,
                 from xfce-titled-dialog.c:41:
/usr/include/glib-2.0/gobject/gtype.h:1356:10: note: declared here
 1356 | void     g_type_class_add_private       (gpointer                    g_class,
      |          ^~~~~~~~~~~~~~~~~~~~~~~~
xfce-titled-dialog.c: In function 'xfce_titled_dialog_init':
xfce-titled-dialog.c:167:13: warning: Deprecated pre-processor symbol: replace with "G_ADD_PRIVATE"
  167 |   titled_dialog->priv = XFCE_TITLED_DIALOG_GET_PRIVATE (titled_dialog);
      |             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~      

EDIT: They could actually have been fixed later, because in this case they are "true deprecation warnings", i.e. triggered by GLIB_VERSION_MIN_REQUIRED, not by GLIB_VERSION_MAX_ALLOWED. But we might as well fix them now.