Move embedded screenshot from to-be-retired wiki.gnome.org to GitLab repository
Per https://discourse.gnome.org/t/wiki-gnome-org-retirement-feedback-wanted/17314/28
Image should probably be moved in GitLab code repository and called via https://gitlab.gnome.org/GNOME/$reponame/-/raw/$branchname/path/to/image/$imagename.png
./gnome-keysign/data/org.gnome.Keysign.raw.appdata.xml: <image width="646" height="529">https://wiki.gnome.org/Apps/Keysign?action=AttachFile&do=get&target=send.png</image>
./gnome-keysign/data/org.gnome.Keysign.raw.appdata.xml: <image width="646" height="529">https://wiki.gnome.org/Apps/Keysign?action=AttachFile&do=get&target=receive.png</image>