authPrompt: Use signal handler instead of vfunc

The prompt itself may get destroyed when canceled, in which
case it is no longer possible to chain up in the vfunc.

This is usually not an issue as the prompt is only destroyed
at the end of a transition, but it results in a warning if
animations are disabled.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3317>

js/gdm/authPrompt.js

@@ -133,10 +133,10 @@ export const AuthPrompt = GObject.registerClass({
         this._userVerifier = null;
     }
 
-    vfunc_key_press_event(event) {
+    on_key_press_event(event) {
         if (event.get_key_symbol() === Clutter.KEY_Escape)
             this.cancel();
-        return super.vfunc_key_press_event(event);
+        return Clutter.EVENT_PROPAGATE;
     }
 
     _initInputRow() {