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.cpp

@@ -500,11 +500,7 @@ MusicBrainzClient::ResultList MusicBrainzClient::UniqueResults(const ResultList
 
   ResultList ret;
   if (opt == UniqueResultsSortOption::SortResults) {
-#if (QT_VERSION >= QT_VERSION_CHECK(5, 14, 0))
     ret = QSet<Result>(results.begin(), results.end()).values();
-#else
-    ret = QSet<Result>::fromList(results).values();
-#endif
     std::sort(ret.begin(), ret.end());
   }
   else {  // KeepOriginalOrder

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_;
 }