opam file: add x-maintenance-intent

[hannesm]

Hey,

would you mind to add a x-maintenance-intent to your opam files, to define which index releases are maintained? I guess a value of [ "(latest)" ] is good for index.

Read more about it at https://github.com/ocaml/opam-repository/blob/master/governance/policies/archiving.md#summary and in more detail https://github.com/ocaml/opam-repository/blob/master/governance/policies/archiving.md#specification-of-the-x--fields-used-in-the-archiving-process

Thanks a lot!