The multimedia-class documentation is critical, because of the large number of changes between Qt 5 and 6. Unfortunately it does not appear to have been maintained.
One error can be found here. It instructs developers to call a constructor that has been deleted:
imageCapture = new QImageCapture(camera);
If you look at the source where the constructor was deleted (qimagecapture.h), you see:
// This is here to flag an incompatibilities with Qt 5
QImageCapture(QCamera *) = delete;