dsa-check-running-kernel: sort kernel versions better

When we ask apt-cache for information on the metapackage to extract the
actual kernel image it depends on, we sort that output.  It's not clear
why we do that, since we pass the option --no-all-versions to apt-cache
so we should only get one answer anyhow.  Nevertheless, since we sort we
might just as well use -V to version sort that output.  dpkg-version
sort would be even better, but hey.  This is based on Erik Auerswald's
patch in #884328.