Add lower-bounds check in CI

[shonfeder]

The last two releases have needed rework due to lower bounds errors detected during package publication. But we can test these in our dev repo using the recipe documented in the opam-repo wiki: https://github.com/ocaml/opam-repository/wiki/Lower-bound-check---DYI#lower-bounds-checks-in-our-ci

This will reduce the need for rework.

[shonfeder]

This should be done on top of #13731, because we need to check the lower bounds of each package spec in isolation: #13731