Add fixed version for CVE-2020-1699/ceph

The CVE was fixed in the experimental upload versioned as 14.2.6-3 which
was actually meant for unstable, but landed in experimental. So the
first version in unstable containing the ix is the following one after
that.

Cf. https://tracker.debian.org/news/1094868/accepted-ceph-1426-4-source-into-unstable/