Use GitHub teams to manage project permissions

The OCaml.org and OCaml Platform teams are working towards making the OCaml Platform governance more transparent (and as a by-product, the structure of the broader ecosystem).

We've recently included the OCaml Platform governance policy (including its lifecycle and the requirements for each stage of the Platform) into the OCaml governance: https://ocaml.org/policies/governance.

As a next step, we're working on a governance page that lists the teams and working groups of the OCaml ecosystem: https://github.com/ocaml/ocaml.org/pull/1239.

The main challenge with this page is to accurately list the members of each team (i.e. the maintainers of each project).

We initially attempted to ask the projects for a list of maintainers, but realised that the outcome had false negatives: the lists were biased towards recently active maintainers.

While our goal was always to move towards using GitHub as a source of truth for the list of maintainers so we don't duplicate the information, we now think that this should be a pre-requisite for the governance page, so as to avoid any bias and use a clear and objective definition of "maintainer": one who has commit access to the repository.

However, GitHub teams are not used uniformly across projects in the OCaml GitHub organisation. The present issue is to request the harmonisation of project permission handling to use GitHub teams for all the projects, so we can use these teams as the source of truth for project maintainers.

At the same time, we'd also like to move a few projects of the OCaml Platform that have reached the Active stage to the OCaml GitHub organisation, as indicated in the Platform governance (namely, utop, ppxlib and opam-publish).

If the request seems sensible, I'll edit the issue to add a TODO list I've prepared for the teams to create and Platform repositories to move to the GH organisation.

Start by reading the OCaml governance policy and the governance page work in PRs 1175 and 1239, then audit how GitHub teams and permissions are currently used across the OCaml organisation. Done would require a concrete TODO list, harmonised team-based project permissions, and the proposed moves of utop, ppxlib, and opam-publish.