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/song.h

@@ -482,11 +482,7 @@ Q_DECLARE_METATYPE(SongMap)
 Q_DECLARE_METATYPE(Song::Source)
 Q_DECLARE_METATYPE(Song::FileType)
 
-#if QT_VERSION >= QT_VERSION_CHECK(6, 0, 0)
 size_t qHash(const Song &song);
-#else
-uint qHash(const Song &song);
-#endif
 // Hash function using field checked in IsSimilar function
 size_t HashSimilar(const Song &song);