drop support for Coq 8.15
We only support the last 2 released Coq versions, currently that is 8.17 and 8.16 (with the 8.18 release being imminent). So let's drop 8.15.
We only support the last 2 released Coq versions, currently that is 8.17 and 8.16 (with the 8.18 release being imminent). So let's drop 8.15.