OpenBSD worker upgraded to OpenBSD 7.9

oregano (the openbsd-amd64 cluster worker) has been upgraded to OpenBSD 7.9.

• Base images rebuilt on 7.9: openbsd-79-amd64-ocaml-4.14 (4.14.4) and openbsd-79-amd64-ocaml-5.5 (5.5.0)
• ocaml-ci updated to request the openbsd-79 images (#1066)