This PR prevents older versions of deb packages from including systemd-unit
files.
Because of the way Krill is set up, this requires both Ubuntu 20.04 and
Debian 11 to have usrmerge (which both have by default, only if upgraded
from an older version it might be missing).
This PR removes the static-openssl feature. If necessary, this features can
be invoked by selecting openssl/vendored directly.
The PR also removes building and testing with default features from the CI
workflow. The only difference between default and all features currently is
rta which is deprecated and will be removed, anyway.
This PR now also fixes a few issues in krillc config simple and the post
install scripts for Debian and RPM packages.
