qtmqtt#dev depends on qtwebsockets which isn't part of Qt 6.0 release

in dependencies.yaml there seems to be

dependencies:
..
*../qtwebsockets:
ref: a0a81f9217a1a197b7b9a4b22edaeae221836b5b
required: false*

If we want to publish qtmqtt (QTBUG-88107) based on Qt 6.0 we need to remove dependency to unsupported module