Js: Use new :icon-name property

Now that we have the new convenience property, apply it to all
icon-only buttons.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2254>

js/ui/unlockDialog.js

@@ -566,7 +566,7 @@ var UnlockDialog = GObject.registerClass({
             opacity: 0,
             x_align: Clutter.ActorAlign.END,
             y_align: Clutter.ActorAlign.END,
-            child: new St.Icon({ icon_name: 'system-users-symbolic' }),
+            icon_name: 'system-users-symbolic',
         });
         this._otherUserButton.set_pivot_point(0.5, 0.5);
         this._otherUserButton.connect('clicked', this._otherUserClicked.bind(this));