Use consistently QT_VERSION to check for KF5 vs. KF6 API
Recent commit 10faae2f also used the Qt version to check for "Are we using KF6 API" for other parts of KCalendarCore::Calendar API to reimplement or not.
More specific might be checking KCALENDARCORE_VERSION, but it can be assumed the check for the Qt version is equivalent :)
In general IMHO it also is better to use the version instead of using the BUILD_DEPRECATED_SINCE
macros at the API consumer side, as that never really was fully supported on the KF side as reliable setting and only fully worked by chance for certain versions. The only place to support it are public headers, to allow the Qt6/KF6 compat builds.
Edited by Friedrich W. H. Kossebau