bump opam to 2.1

[gares]

@erikmd opam 2.1 has the following advantages:

• automatically installs depext, see for example a few basic lemmas about ereal math-comp/analysis#609 (comment) and the resulting fix
• much faster in resolving constraints involving hierarchy builder [question] Howto debug/fix slow constraint solving ocaml/opam#4495

Fixes: coq-community/docker-coq#38

[erikmd]

Hi @gares, thanks for your PR! sorry for replying a bit late, being swamped these days.

But actually, it can't work as is, because of this issue:

ocaml/opam.ocaml.org#24
ocaml/opam.ocaml.org#23

Because of that, it is not possible to build new coqorg/base images since a few months, because the verification of the opam binary requires the availability of the PGP public key which still returns a 404:

$ curl -fsSL https://opam.ocaml.org/opam-dev-pubkey.pgp
curl: (22) The requested URL returned error: 404

I will ping the opam team anew then.

As soon as the problem above is solved, the docker-base CI will become green again thanks to its nightly build pipeline:

[erikmd]

@gares so the coqorg/base builds were fine; but I now have to be away from keyboard;
I'll try to propagate this new opam version tonight to all children images (rebuilding/monitoring coqorg/coq and mathcomp…).

[gares]

Thanks, no pressure really.