Add support for OCaml 5.5

[kit-ty-kate]

See ocaml/ocaml#14224
Queued on #6684

TODO:

• add a way to test compiler betas
• add OCaml 5.4 to the test matrix

[kit-ty-kate]

To expend a little bit on the TODOs listed above, as done in yesterday's dev meeting:

During each OCaml release cycle, making sure opam compiles with it on each platform we test is extremely time-consuming without taking advantage of our CI. However currently it doesn't support pre-releases of the compiler without some heavy hacks (and even then...). With my OCaml Software Foundation hat on, testing it is also extremely useful for the compiler itself (e.g. ocaml/ocaml#13541) and it is thus it is more useful during the pre-releases.

I'm thus planning to modify the CI scripts to be able to test pre-releases of the compiler, and maybe compiler PRs too (although to avoid hacks i'd need ocaml/opam-repository#28455, so maybe in a separate PR)