Support for old versions of OCaml (≤ 3.12)

[thierry-martinez]

setup-ocaml doesn't work with old versions of OCaml (≤ 3.12). (It was already noted in the issue https://github.com/ocaml/opam-repository/issues/22015 which was transferred to opam-repository, but this part of the issue seems to be specific to setup-ocaml.)

• With 3.07, it fails with the message Error: No OCaml base compiler packages matched the version 3.07 in the opam-repository., whereas opam switch create 3.07 should work as expected (https://github.com/thierry-martinez/stdcompat/runs/7952973003?check_suite_focus=true).
• With OCaml 3.08 to 3.12, it seems to fail because it tries to install opam-depext which requires OCaml ≥ 4.00.