Update wallpaper for edge

Use the "meadow" wallpaper (light and dark) that was added in https://gitlab.com/postmarketOS/artwork/-/merge_requests/25 by @dikasp in edge, and backport that change with v23.12 SP 1 (#2428 (closed)). Moving forward, the idea is to change the wallpaper with every release, now that we have a few very nice ones to choose from!

From discussions in artwork!25 (closed), the team, and with Sxmo, it looks like people would be fine if we use this for all UIs. The nice thing then is of course, that it is consistent!

If somebody wants to take this over, feel free to do so. Otherwise I'll try to get to it some time soon.