Use JSON metadata for Aurorae SVG themes
Generally we are moving towards using JSON file metadata instead of Desktop file metadata for packages.
For QML-based Aurorae themes we already prefer metadata.json and warn when metadata.desktop is used: https://invent.kde.org/plasma/kwin/-/blob/master/src/plugins/kdecorations/aurorae/src/aurorae.cpp#L759
However for SVG-based themes we only support metadata.desktop: https://invent.kde.org/plasma/kwin/-/blob/master/src/plugins/kdecorations/aurorae/src/aurorae.cpp#L664
We probably should move towards using JSON there too, if only for consistency