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/subsonic/subsonicbaserequest.cpp

@@ -107,10 +107,7 @@ QNetworkReply *SubsonicBaseRequest::CreateGetRequest(const QString &ressource_na
 
   req.setHeader(QNetworkRequest::ContentTypeHeader, QStringLiteral("application/x-www-form-urlencoded"));
   req.setAttribute(QNetworkRequest::RedirectPolicyAttribute, QNetworkRequest::NoLessSafeRedirectPolicy);
-
-#if QT_VERSION >= QT_VERSION_CHECK(5, 15, 0)
   req.setAttribute(QNetworkRequest::Http2AllowedAttribute, http2());
-#endif
 
   QNetworkReply *reply = network_->get(req);
   QObject::connect(reply, &QNetworkReply::sslErrors, this, &SubsonicBaseRequest::HandleSSLErrors);