ci: check ocaml 5

[Khady]

Make sure that devkit compiles on ocaml 5.

For now it depends on a unreleased version of libevent.

There is also a warning that would be nice to fix, but it's not blocking:

File "httpev.ml", line 146, characters 33-46:
146 |     match Unix.getsockopt_int fd Unix.SO_ERROR with
                                       ^^^^^^^^^^^^^
Alert deprecated: SO_ERROR
Use Unix.getsockopt_error instead.

File "httpev.ml", line 819, characters 57-70:
819 | let check_req req = match Unix.getsockopt_int req.socket Unix.SO_ERROR with 0 -> `Ok | n -> `Error n
                                                               ^^^^^^^^^^^^^
Alert deprecated: SO_ERROR
Use Unix.getsockopt_error instead.