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/settings/subsonicsettingspage.cpp

@@ -60,11 +60,7 @@ SubsonicSettingsPage::SubsonicSettingsPage(SettingsDialog *dialog, QWidget *pare
 
   dialog->installEventFilter(this);
 
-#if QT_VERSION >= QT_VERSION_CHECK(5, 15, 0)
   ui_->checkbox_http2->show();
-#else
-  ui_->checkbox_http2->hide();
-#endif
 
 }