WebAPI: fix wrong behavior for shutdown action

Qt6 has changed implementation for `QCoreApplication::quit` and therefore it is not suitable
anymore.

Closes #17709.
PR #17720.

src/webui/api/appcontroller.cpp

@@ -91,12 +91,13 @@ void AppController::buildInfoAction()
 
 void AppController::shutdownAction()
 {
-    qDebug() << "Shutdown request from Web UI";
+    // Special handling for shutdown, we
-
-    // Special case handling for shutdown, we
     // need to reply to the Web UI before
     // actually shutting down.
-    QTimer::singleShot(100ms, qApp, &QCoreApplication::quit);
+    QTimer::singleShot(100ms, qApp, []()
+    {
+        QCoreApplication::exit();
+    });
 }
 
 void AppController::preferencesAction()