- In .gitlab-ci.yml, support both Qt5 and Qt6 builds
- In .kde-ci.yml, use different libraries (or versions thereof)
  depending on Qt5 or Qt6
- CMakeLists.txt chooses between Qt5 and Qt6 depending on environment
  variables QT_MAJOR_VERSION, BUILD_WITH_QT6, or BUILD_WITH_QT5 or
  whatever Qt version is automatically detected