chore: drop Qt 5 support

The required exiv2 version is now 0.28.x as well since it was
keeped for Qt 5 build.

Due to exiv2ConfigVersion.cmake doesn't allow us write something
like find_package(exiv2 "0.28") to require version >= 0.28.0,
we simply don't put the required version number here for now.

app/main.cpp

@@ -24,9 +24,6 @@ int main(int argc, char *argv[])
     QGuiApplication::setHighDpiScaleFactorRoundingPolicy(Settings::instance()->hiDpiScaleFactorBehavior());
 
     QApplication a(argc, argv);
-#if QT_VERSION < QT_VERSION_CHECK(6, 0, 0)
-    a.setAttribute(Qt::ApplicationAttribute::AA_UseHighDpiPixmaps);
-#endif
 
     QTranslator translator;
 #if defined(TRANSLATION_RESOURCE_EMBEDDING)