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/playlist/playlisttabbar.cpp

@@ -390,11 +390,7 @@ void PlaylistTabBar::dragEnterEvent(QDragEnterEvent *e) {
 
 void PlaylistTabBar::dragMoveEvent(QDragMoveEvent *e) {
 
-#if QT_VERSION >= QT_VERSION_CHECK(6, 0, 0)
   drag_hover_tab_ = tabAt(e->position().toPoint());
-#else
-  drag_hover_tab_ = tabAt(e->pos());
-#endif
 
   if (drag_hover_tab_ != -1) {
     e->setDropAction(Qt::CopyAction);