Convert png to webp

gnome-backgrounds has switched to webp as the format for its wallpapers. I think Plasma should also do the same" for all the backgrounds in this repository.

Ref: https://gitlab.gnome.org/GNOME/gnome-backgrounds/-/issues/29 https://gitlab.gnome.org/GNOME/gnome-build-meta/-/issues/502 https://pagure.io/fedora-workstation/issue/327