appdata verification seems to fail with a webp screenshot

We seem to still need a .png-screenshot somewhere to make the appdata/appstream verification happy (this currently blocks flatpak releases).

.webp files seem to be invalid:

/build/flatpak# flatpak run --env=G_DEBUG=fatal-criticals org.freedesktop.appstream-glib validate build-dir/*/share/appdata/io.gitlab.librewolf-community.appdata.xml
build-dir/files/share/appdata/io.gitlab.librewolf-community.appdata.xml: FAILED:
? file-invalid          : <screenshot> failed to load [https://librewolf.net/screenshot-light.webp]
Validation of files failed

This MR is just the screenshot-light.webp converted to a .png.