end_dialog.gd "popup_confirm" not existed anymore

After the creation of the RGT_ConfirmationDialog, the method popup_confirm does not exist anymore.

Possible fix
```gdscript
func _on_restart_button_pressed():
  confirm_dialog.dialog_text = confirm_restart
  confirm_dialog.force_ok_pressed_callable(_on_confirm_restart_confirmed)
  confirm_dialog.popup_centered()
```