Add x-maintenance-intent to dune opam files

[art-w]

Using #11274 to add it to every opam files. My understanding is that only the "(latest)" version is the one actively maintained, but it's easy to change if you want to include more versions.

[Leonidas-from-XIV]

The OPAM files are generated automatically from dune-project so unless the change is added in the respective .template files, the change will be removed on the next dune build. I suggest landing #11274 first and then adding maintenance_intent to our dune-project in this PR, which will automatically update the opam files.