add x-maintenance-intent

[mseri]

Due to the opam repository archival process, we are keen to have the maintainers of packages specify their intention. It would be useful, also to ensure a smooth experience for the users, to clarify owl's maintenance intent.

Looking at the past and new releases, I think the most sensible setting is that only the latest released is considered maintained, but I am happy to change this if you disagree. You can read up on the policy at https://github.com/ocaml/opam-repository/blob/master/governance/policies/archiving.md and the x-maintenance-intent field at https://github.com/ocaml/opam-repository/blob/master/governance/policies/archiving.md#specification-of-the-x--fields-used-in-the-archiving-process