CI: multiple OCaml versions

[liyishuai]

Prevent future versions of #1022

[JasonGross]

You need to update

concurrency:
      group: ${{ github.workflow }}-${{ matrix.target }}-Ubuntu-${{ github.event_name }}-${{ github.head_ref || github.run_id }}

to include ${{ matrix.ocaml_version }} to avoid one ocaml version cancelling the others

[mattam82]

I'd rather have just 4.09 than all the intermediate version, this is somewhat costly otherwise.