Remove opam 2.0 support The service config pins V2_5 for every build platform and lint.ml uses V2_2; nothing constructs V2_0, so the V2_0 cases in Variant.pp, the opam_build depext match, and Opam_version.of_string were never executed. Variant.pp also drops the V2_0-as-empty-suffix special case, so all variants now print with an explicit _opam-X.X suffix. That in turn makes two more pieces of API redundant: - Opam_version.default -- referenced only by Variant.of_string as a fallback for un-suffixed variant strings. Now that nothing emits un-suffixed strings, of_string rejects them outright. - Opam_version.to_string_with_patch -- declared in the .mli, called nowhere. Prepares for ocurrent/docker-base-images#342 and upstream ocurrent/ocaml-dockerfile#262 to remove the opam-2.0 base images. Deploy-safe in any order: V2_5 platforms pull the same _opam-2.5 tag before and after, and V2_0 was already unused.