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/osdMonitorLabeler.js

@@ -14,7 +14,7 @@ class OsdMonitorLabel extends St.Widget {
         this._monitor = monitor;
 
         this._box = new St.BoxLayout({
-            vertical: true,
+            orientation: Clutter.Orientation.VERTICAL,
         });
         this.add_child(this._box);