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/graphicsview.cpp

@@ -41,9 +41,7 @@ void GraphicsView::showFileFromPath(const QString &filePath)
         QImageReader imageReader(filePath);
         imageReader.setAutoTransform(true);
         imageReader.setDecideFormatFromContent(true);
-#if QT_VERSION > QT_VERSION_CHECK(6, 0, 0)
         imageReader.setAllocationLimit(0);
-#endif //QT_VERSION > QT_VERSION_CHECK(6, 0, 0)
 
         // Since if the image format / plugin does not support this feature, imageFormat() will returns an invalid format.
         // So we cannot use imageFormat() and check if it returns QImage::Format_Invalid to detect if we support the file.