Drop Qt 5 support

Qt 6 has been available for almost 4 years. Qt 5 is no longer officially supported by Qt for opensource, it's time to drop Qt 5.

src/core/standarditemiconloader.cpp

@@ -92,11 +92,7 @@ void StandardItemIconLoader::RowsAboutToBeRemoved(const QModelIndex &parent, int
 
 void StandardItemIconLoader::ModelReset() {
 
-#if (QT_VERSION >= QT_VERSION_CHECK(5, 14, 0))
   cover_loader_->CancelTasks(QSet<quint64>(pending_covers_.keyBegin(), pending_covers_.keyEnd()));
-#else
-  cover_loader_->CancelTasks(QSet<quint64>::fromList(pending_covers_.keys()));
-#endif
   pending_covers_.clear();
 
 }