Retiring opam 2.0 from the build pipeline

ocurrent/docker-base-images publishes the ocaml/opam:* Docker images which the OCaml CI systems use. For each distro, it tracks 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, and master opam release branches in parallel and produces both an opam-version-suffixed tag (e.g. debian-13-ocaml-5.4_opam-2.5) and an un-suffixed default that points at the oldest tracked version.