message: fix: withdraw reg via pending_withdrawal
  Never set `withdrawn` directly, let the backend handle it.
diff: @@ -19,7 +19,7 @@ export default class RegistrationIsPublic extends Component.extend({
  }
  this.node.setProperties({
  - withdrawn: true,
  + pendingWithdrawal: true,
  withdrawalJustification: this.withdrawalJustification,
  });
  