drop support for Coq 8.15

After this landed we should finally be able to make use of https://github.com/coq/coq/pull/13969. :)

.gitlab-ci.yml

@@ -74,16 +74,9 @@ build-coq.8.17.1:
     DENY_WARNINGS: "1"
     MANGLE_NAMES: "1"
 
+# The oldest version runs in MRs, without name mangling.
 build-coq.8.16.1:
   <<: *template
   variables:
     OPAM_PINS: "coq version 8.16.1"
     DENY_WARNINGS: "1"
-
-# The oldest version runs in MRs, without name mangling.
-build-coq.8.15.2:
-  <<: *template
-  <<: *branches_and_mr
-  variables:
-    OPAM_PINS: "coq version 8.15.2"
-    DENY_WARNINGS: "1"