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/musicbrainz/musicbrainzclient.h

@@ -212,11 +212,7 @@ class MusicBrainzClient : public QObject {
 
 };
 
-#if QT_VERSION >= QT_VERSION_CHECK(6, 0, 0)
 inline size_t qHash(const MusicBrainzClient::Result &result) {
-#else
-inline uint qHash(const MusicBrainzClient::Result &result) {
-#endif
   return qHash(result.album_) ^ qHash(result.artist_) ^ result.duration_msec_ ^ qHash(result.title_) ^ result.track_ ^ result.year_;
 }