Prepare support for common linux-qt6 CI template

Replace CMake variable QT6 with common BUILD_WITH_QT6 and QT_MAJOR_VERSION variables:

• https://invent.kde.org/sysadmin/ci-utilities/raw/master/gitlab-templates/linux-qt6.yml
• https://invent.kde.org/frameworks/extra-cmake-modules/-/raw/master/modules/QtVersionOption.cmake

Keep checking the QT6 variable for compatibility reasons.