Collect Iris versions required by artifacts
I'd like to clean up https://gitlab.mpi-sws.org/iris/opam, to avoid opam issues due to having too many versions of a package. However, it turns out in the past some of the Iris paper artifacts are not self-contained, and instead rely on the opam repository. We should figure out which versions of Iris are required by an artifact, and keep them in the repository.