opam-depext 1.2 for opam 2.1.0+

[dra27]

The opam-depext plugin is disabled for opam 2.1, but this creates a slight CLI issue originally tracked in ocaml/opam#4778.

Conclusion is that we'll release a version of opam-depext which is only for opam 2.1.0+ which will invoke opam with --no-depexts. The package itself will warn opam 2.1 users that they shouldn't have installed it (since it will continue to carry the major wart of opam 2.0 in that it calls the solver twice and may result in a different solution).