[PATCH 0/3] Improve CI testing of older systems

In many cases, we're not testing some of the older Linux distributions that we support. However, we _are_ testing some old Linux distributions that we _don't_ support. That means that it's presently difficult for us to make sure our code works on all the systems we want it to and that we're not accidentally requiring newer versions of dependencies than we claim to support.

The first patch removes Ubuntu 16.04, which is out of regular LTS support. The second removes the clause in the CI job for that OS. The third adds several new CI jobs to make sure we're adequately supporting older Debian and RHEL-compatible distros, and documents the dates at which they lose support (so it's easy to know if we need to update them).

brian m. carlson (3):
  gitlab-ci: switch from Ubuntu 16.04 to 20.04
  ci: remove clause for Ubuntu 16.04
  Add additional CI jobs to avoid accidental breakage

 .github/workflows/main.yml |  9 +++++++++
 .gitlab-ci.yml             |  2 +-
 ci/install-dependencies.sh | 19 ++++++++++---------
 3 files changed, 20 insertions(+), 10 deletions(-)