Switch to ocaml-ci

[jmid]

https://github.com/ocurrent/ocaml-ci is coming along nicely and offers tests on a range of platforms and OCaml versions,
so we might consider switching to it.

Advantages:

• works with the OCaml-ecosystem, meaning less workflow files to maintain (looking at you Update GitHub actions #275)
• more platform combinations will be exercised, allowing to build confidence in a PR
• non-portable issues will be caught early rather than when I bang my head against the errors from opam-ci at release time

Disadvantages:

• no Windows testing ATM AFAIK (we would keep a Windows GitHub actions workflow for that though)