dune-project / generate-opam-files: add the x-maintenance-intent field

[hannesm]

Dear Madam or Sir,

since the opam-repository is growing, and doesn't scale - there is now a package archival policy relying on an opam field named x-maintenance-intent.

The discussion on the archival resulted that we want package authors to specify how their opam packages are maintained. By default, nothing will be removed. And even if some package version is not maintained, but required by another package, it will be kept in the opam-repository.

By default, all versions are kept -- so we rely on package authors to include that field.

Here, dune comes into the game: it would be great if dune-project when generating opam files could embed this field, and the dune-project template could contain that field (x-maintenance-intent: [ "(latest)" ] is the most likely choice).

See further documentation on that field at https://github.com/ocaml/opam-repository/blob/master/governance/policies/archiving.md#specification-of-the-x--fields-used-in-the-archiving-process

Thanks for your cooperation, and your contribution to the OCaml ecosystem.

[maiste]

Thanks for report! This is covered by @art-w in #11274