Proper OCaml 5 CI

[Gau-thier]

@gsebil08 Since the goal is to use this via deku I would rather have ocaml 5 in esy and then test other versions in opam

Originally posted by @ulrikstrid in #29 (comment)

Goal: fit with the Deku CI

[rosalogia]

Closed by #29

[Gau-thier]

Actually @rosalogia, the CI done in #29 is "good enough".
It add the OCaml 5 scenario, and get rid of the 4.08, but since our main client will be Deku, @ulrikstrid said it would be better to have the same CI scenarii.
Hence, I re-open this one, to work on it later.