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/covermanager/albumcovermanagerlist.cpp

@@ -42,11 +42,7 @@
 
 AlbumCoverManagerList::AlbumCoverManagerList(QWidget *parent) : QListWidget(parent), manager_(nullptr) {}
 
-#if QT_VERSION >= QT_VERSION_CHECK(6, 0, 0)
 QMimeData *AlbumCoverManagerList::mimeData(const QList<QListWidgetItem*> &items) const {
-#else
-QMimeData *AlbumCoverManagerList::mimeData(const QList<QListWidgetItem*> items) const {
-#endif
 
   // Get songs
   SongList songs;