Metatask to migrate to Qt6. Steps are:
* Migrate to Qt5.12 as minor version if needed because "old" supported distributions has not yet changed to Qt5.15 minimal.
* Migrate to Qt5.15 :
* integrate https://phabricator.kde.org/T10168 / https://invent.kde.org/education/gcompris/-/merge_requests/78
* to be able to use Version-less CMake targets (see https://www.qt.io/blog/versionless-cmake-targets-qt-5.15 and https://doc.qt.io/qt-6/portingguide.html)
* Migrate to Qt6.2.x: POC in branch work/poc/qt6 (started from the calendar_no_qtquick branch to already have QtQuick Controls2). It would be complicated to keep the cohabitation between Qt5 and Qt6 due to qml)