Add maintenance-intent in dune-project file

[balat]

This is a request from the opam-repository maintainers.

The ultimate goal is to push package versions that are not maintained (and thus irrelevant) to a opam-repository-archive - which will make every client have to perform fewer work, every CI system to compute less. Apart from CPU and memory savings, there's bandwidth being saved.

policy document https://github.com/ocaml/opam-repository/blob/master/governance/policies/archiving.md