aftrer upgrade to Qt6, remove Qt version switches

like in GUI/View/Projection/SaveProjectionsAssistant.cpp:

#if QT_VERSION >= QT_VERSION_CHECK(5, 14, 0)

etc

added cleanup label

created branch 304-aftrer-upgrade-to-qt6-remove-qt-version-switches to address this issue

mentioned in commit 429ee5e4

mentioned in merge request !1006 (merged)

mentioned in commit 730ef08b

closed with merge request !1006 (merged)