Stop QImageReader from asserting when creating large thumbnails for KRA (in Qt6)

I hit this while Krita is creating thumbnails for some of my larger KRA files under Qt6. To be more specific, this is because it picks "mergedimage.png" which is quite large instead of the smaller res "preview.png" which seems to be intended.

Anyway, this is probably better to disable for now to match Qt5 behavior and re-enable later down the line.

Test Plan

• Find a KRA file with a large enough mergedimage.png or other thumbnail-suitable image used by KisFileIconCreator (I can try to create one if needed)
• Try to load it in the recent documents view
• Krita shouldn't assert with the message qt.gui.imageio: QImageIOHandler: Rejecting image as it exceeds the current allocation limit of 256 megabytes

Formalities Checklist

• I confirmed this builds.
• I confirmed Krita ran and the relevant functions work.
• I tested the relevant unit tests and can confirm they are not broken. (If not possible, don't hesitate to ask for help!)
• I made sure my commits build individually and have good descriptions as per KDE guidelines.
• I made sure my code conforms to the standards set in the HACKING file.
• I can confirm the code is licensed and attributed appropriately, and that unattributed code is mine, as per KDE Licensing Policy.
• Does the patch add a user-visible feature? If yes, is there a documentation MR ready for it at Krita Documentation Repository?

Reminder: the reviewer is responsible for merging the patch, this is to ensure at the least two people can build the patch. In case a patch breaks the build, both the author and the reviewer should be contacted to fix the build. If this is not possible, the commits shall be reverted, and a notification with the reasoning and any relevant logs shall be sent to the mailing list, kimageshop@kde.org.