Update hosts to Ubuntu Bionic (18.04)
Currently our production fleet runs on Ubuntu Xenial (16.04). It reached the end of its "Hardware and maintenance updates" life at the end of 2018 (see https://www.ubuntu.com/about/release-cycle). In addition, we've started to be missing newer versions of packages we want to update to on Xenial's official repositories (e.g. #5293 (comment 158633721)).
We should update to 18.04, the LTS version currently fully suported.