Default wallpaper switched to JXL for Fedora 42 Beta

[Luya Tshimbalanga]

Hello everyone,

Design team switched to JXL for the Fedora 42 default wallpaper, which 
may impact some spins. For beta release, it will be nice
for each spin to adopt JXL format as soon as possible. Alternatively, 
those wishing to keep PNG requires a conversion upstream from
JXL to PNG[1]. Details is on 
https://fedoraproject.org/wiki/Changes/SwitchToJXLforDefaultWallpaper

Regards,

Reference:

[1] https://github.com/fedoradesign/backgrounds/issues