Bump Iris version to get more recent stdpp version (blocking coq/coq#16920)

Coq-ci is using coq-iris-heap-lang version dev.2023-03-10.0.45e5a052 through coq-iris-examples.opam. What would be a good choice of new version to take benefits from changes in std++ which look to be integrated in iris in commit 80e0a15f? This is blocking PR coq/coq#16920.

@proux