Refs #37034 - Use apt-key add on Ubuntu 16.04

[maximiliankolb]

On Ubuntu 16.04, which is supported by Canonical until 2026, you cannot place a GPG public key in "/etc/apt/trusted.gpg.d/" and use it to verify the signature of "Release" files.

EoL see https://ubuntu.com/16-04

Ubuntu 14.04 is no longer supported by Canonical; Ubuntu 18.04 and all supported Debian version can handle GPG public keys in "/etc/apt/trusted.gpg.d/".

Test on Ubuntu 16.04 in Container (ignore the repository and package; this is just to ensure that the package installation works):

FROM ubuntu:xenial

RUN apt-get update && \
    apt-get install -y curl ca-certificates pgpgpg && \
    curl --silent --show-error https://oss.atix.de/atix_gpg.pub | apt-key add - && \
    mkdir -p /etc/apt/sources.list.d/ && \
    echo "deb http://oss.atix.de/Ubuntu20LTS/ stable main" > /etc/apt/sources.list.d/client.list && \
    apt-get update && \
    apt-get install -y apt-transport-katello