Bump MathComp lower bound to >= 1.16.0 and Coq to >= 8.16.

Currently a draft as I need to update the OPAM package first. (https://github.com/coq/opam-coq-archive/pull/2516 merged)

I also don't know how to update the Docker image bbbrandenburg/alectryon-ci:1.14.0-coq-8.15.0 to some bbbrandenburg/alectryon-ci:1.16.0-coq-8.16.0

Fixes: #85 (closed)