[REG 5.15.x -> 6.x] Indexed images become dithered after scaling them

Reproducing this is as simple as converting any image (I have checked using a screenshot of my desktop) to the Indexed8 format using e.g. GIMP (https://docs.gimp.org/en/gimp-image-convert-indexed.html) then trying to scale it with QImage::scaled (personally I tried 1280x1280 with Qt::KeepAspectRatio, from 1920x1080) using Qt::SmoothTransforation (I haven't checked without the flag though). In Qt 6, the image becomes dithered, while in Qt 5 is not. Reverting b0b4fcd05cd5ae891da868829c1645acf7589acb fixes the issue, just removing convertToFormat(format()) after smoothScaled also fixes the issue.