drop support for Coq 8.16, 8.17
With Coq 8.19 released, we are only promising to support 8.18 and 8.19, so we can drop the older ones. This is needed to unblock stdpp!537 (merged).
With Coq 8.19 released, we are only promising to support 8.18 and 8.19, so we can drop the older ones. This is needed to unblock stdpp!537 (merged).