Align ocaml-base-compiler version.

[folone]

Problem

ocaml-base-compiler version was updated in 21579b5. Since then, it looks like the github builds have been failing with

  [ERROR] No solution for base:   * No agreement on the version of ocaml-base-compiler:
              - (invariant) → ocaml-base-compiler = 4.14.0
              - base → ocaml >= 5.1.0 → ocaml-base-compiler >= 5.1.0~
              You can temporarily relax the switch invariant with `--update-invariant'
            * Missing dependency:
              - base → ocaml >= 5.1.0 → ocaml-variants >= 5.1.0~ → system-msvc
              unmet availability conditions: 'os = "win32"'

Solution

Update the ocaml version to 5.
As a drive-by, updating the setup-ocaml gh action version to v3, since v2 did not support windows os for ocaml 5.
Another drive-by is updating the checkout action version, to keep this workflow up-to-date.