workspace-indicator: Set BoxLayout orientation

Use the new `orientation` property instead of `vertical`, as the
latter is deprecated.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/380>

extensions/workspace-indicator/workspaceIndicator.js

@@ -132,7 +132,7 @@ class WorkspaceThumbnail extends St.Button {
         const box = new St.BoxLayout({
             style_class: 'workspace-box',
             y_expand: true,
-            vertical: true,
+            orientation: Clutter.Orientation.VERTICAL,
         });
         this.set_child(box);