Update information on CVE-2020-0569 and CVE-2020-0570

For CVE-2020-0570 Lisandro asked back to upstream about confirmation on
the affected ranges. Upstream confirmed that the issue is not present
before 5.12:

> The patch just make sure that we don't do wrong call when the search prefixes
> contains '/'
> But before 5.12 (commit 5219c37f7c98f37f078fee00fe8ca35d83ff4f5d), there were
> no search prefixes with '/' in them.
> So no need to apply the patch in earlier versions.

Remove as well the now uneeded TODO item from CVE-2020-0569, as the
issue does not apply to the old qt4-x11.