CI: Add OCaml 5.3 as experimental build

[robhoes]

No description provided.

[robhoes]

The new ocaml_version parameter on setup-xapi-environment causes some of the "expected" checks to be out of sync again... That needs to be fixed in the github config if this PR is approved. The "OCaml tests" are all passing, for 4.14.2 as well as 5.3.0.

[psafont]

At this point it's probably the right thing to drop the sourcing of https://github.com/xapi-project/xs-opam/blob/master/tools/xs-opam-ci.env in .github/workflows/setup-xapi-environment/action.yml
That's how the ocaml version used to be controlled in CI

[psafont]

We need to change the required checks because a new parameter got added to the matrix before merging this. Now there's a check that will never run, an another that runs should be required.

We probably want to merge the open feature PRs before doing this