appdata: add the latest two releases
I was working on the open PR to bring gThumb on Flathub and after building it and installing I noted that it was showing an old version, because the appdata file is not up-to-date.
This simple patch adds the last stable and development releases to the appdata/metainfo file.
I've also considered using the appstreamcli news-to-metainfo
script instead, but eventually I think it's not worth.
I'm open to discuss this matter though.