Update net-im/kaidan to latest upstream release

Upstream switched where it places the tarballs (from "stable/" to
"unstable/" even while releasing stability-fixes), so this wasn't
spotted earlier. While here, update the WWW to point to an actual
site, rather than the unofficial GitHub mirror of the source code.

Release notes at:
https://www.kaidan.im/2020/08/20/kaidan-0.6.0/
https://www.kaidan.im/2021/02/02/kaidan-0.7.0/