Use versionless QtQuick imports

It ensures that we pull the latest features introduced in QtQuick in Qt6.

org.kde.kirigami imports have not been adjusted because otherwise QML
fails to recognize Kirigami. It's most likely connected to the fact that
Kirigami installs its stuff in ${KDE_INSTALL_QMLDIR}/org/kde/kirigami.2
rather than ${KDE_INSTALL_QMLDIR}/org/kde/kirigami