ECMAddQch: support doc linking also against Qt6 docs
With this in, all the ecm_add_qch calls in the KF modules in master also need work to adapt and fully work again, by adapting the LINK_QCHS arguments accordingly to now use Qt6**_QCH.
With this in, all the ecm_add_qch calls in the KF modules in master also need work to adapt and fully work again, by adapting the LINK_QCHS arguments accordingly to now use Qt6**_QCH.