js: Set BoxLayout orientation

Use new `orientation` property instead of `vertical`.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3614>

js/ui/status/backlight.js

@@ -109,7 +109,7 @@ const DiscreteItem = GObject.registerClass({
     _addLevelButton(key, labelText, iconName) {
         const box = new St.BoxLayout({
             style_class: 'keyboard-brightness-level',
-            vertical: true,
+            orientation: Clutter.Orientation.VERTICAL,
             x_expand: true,
         });