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
./brasero/data/brasero.appdata.xml.in: <screenshot type="default">https://wiki.gnome.org/Apps/Brasero?action=AttachFile&do=get&target=copy-disc.png</screenshot>
./brasero/data/brasero.appdata.xml.in: <screenshot>https://wiki.gnome.org/Apps/Brasero?action=AttachFile&do=get&target=audio-disc.png</screenshot>
./brasero/data/brasero.appdata.xml.in: <screenshot>https://wiki.gnome.org/Apps/Brasero?action=AttachFile&do=get&target=data-project.png</screenshot>