Appdata fixes for gnome-42 branch

The screenshots are in an appdata firectory (not metainfo) on the gnome-42 branch, which should stay only on gnome-42. On the other hand, updating the homepage URLs can be cherry-picked to main without problems. Both of these minor issues came up while trying to run appstreamcli validate which with these fixes works except for not understanding the GNOME release versioning scheme.