Fail CI when migrations are targetting the wrong version

[ewjoachim]

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It would be nice if the CI could fail when a PR contains migration files whose name reference a procrastinate version (a.k.a git tag) that is not the latest. This would avoid things like #250.

This would probably either be a test implemented in pytest, or a short script in a new bin folder, alongside a tox target or integrated in check-lint. Whatever feels the most natural to the implementer.