Improve/fix CI and update supported versions of OCaml

This PR improves and fixes the CI and updates the supported versions of OCaml. In particular, it:

runs CI only on pushes to main, tags and pull requests.
adds Dependabot to keep our actions up-to-date.
adds jq as a system dependency in Docker images.
disables cleanup in apt-based CI because it seems broken for this specific version of Dune.
disables building examples in docker-based Debian and openSUSE CI. cf #139.
adds newer versions of OCaml in the CI; making them officially supported (4.12, 4.13, 4.14 and 5.0).
removes versions of OCaml prior to 4.11, which is the version of OCaml currently in Debian stable.

It is based on top of #137 that fixes the test engine, without which CI makes little sense. As such, it should be merged after #137.

colis-anr / morbig