Align `ocaml-base-compiler` version.

[folone]

ocaml-base-compiler version was updated in https://github.com/janestreet/base/commit/21579b5903c81be975591b0a9f12699be8380594. 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"'