ECMAddQtDesignerPlugin: use correct plugin IID
The wrong one seems to have worked somehow still with Qt5's designer, but does not with Qt6's one.
Qt 5.9 doc already hinted to use this one: https://doc.qt.io/archives/qt-5.9/qdesignercustomwidgetcollectioninterface.html Qt 6 doc does as well: https://doc.qt.io/qt-6/qdesignercustomwidgetcollectioninterface.html