chore: 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.

[Leonidas-from-XIV]

I think some of these changes need to be discussed because while we do release them as packages I don't think all of them are meant to be used by others.

e.g. I don't think that we would do a patch-release for a bug in stdune if it doesn't affect the dune package. We'd probably fix it upstream to prevent it becoming an issue in the future but a lot of these packages are really just to support Dune.

[Leonidas-from-XIV]

@rgrinberg Can you take a look and confirm that this matches our maintenance policy?

[maiste]

I encounter an issue while working on this PR. To be able to have the maintenance field in dune-project, I need to increase the number of the lang to 3.18. If I do so, it will build the maintenance field, but it will complain that it can't install dune >= 3.18 because this version does not exist in the local switch test.

Also, it seems that the 3.18 version is the one that changed the formatting, so it also requires the formatting to change. Should it be done in a separated PR if possible?

[Leonidas-from-XIV]

With regards to the switch test, the issue seems to be that the packages are getting pinned to 3.17.1 instead of 3.18:

xdg is now pinned to git+file:///home/runner/work/dune/dune#HEAD (version 3.17.1)
stdune is now pinned to git+file:///home/runner/work/dune/dune#HEAD (version 3.17.1)
ordering is now pinned to git+file:///home/runner/work/dune/dune#HEAD (version 3.17.1)
ocamlc-loc is now pinned to git+file:///home/runner/work/dune/dune#HEAD (version 3.17.1)
dyn is now pinned to git+file:///home/runner/work/dune/dune#HEAD (version 3.17.1)
dune is now pinned to git+file:///home/runner/work/dune/dune#HEAD (version 3.17.1)
dune-site is now pinned to git+file:///home/runner/work/dune/dune#HEAD (version 3.17.1)
dune-rpc is now pinned to git+file:///home/runner/work/dune/dune#HEAD (version 3.17.1)
dune-rpc-lwt is now pinned to git+file:///home/runner/work/dune/dune#HEAD (version 3.17.1)
dune-private-libs is now pinned to git+file:///home/runner/work/dune/dune#HEAD (version 3.17.1)
dune-glob is now pinned to git+file:///home/runner/work/dune/dune#HEAD (version 3.17.1)
dune-configurator is now pinned to git+file:///home/runner/work/dune/dune#HEAD (version 3.17.1)
dune-build-info is now pinned to git+file:///home/runner/work/dune/dune#HEAD (version 3.17.1)
dune-action-plugin is now pinned to git+file:///home/runner/work/dune/dune#HEAD (version 3.17.1)
chrome-trace is now pinned to git+file:///home/runner/work/dune/dune#HEAD (version 3.17.1)

But I do agree that this PR probably needs to be split into two, one that bumps to 3.17 and adds the formatting and one that bumps to 3.18 and updates the maintenance intent.

[maiste]

You are right, let's do it this way 👍