CI: bump preferred Coq version

.gitlab-ci.yml

@@ -29,7 +29,7 @@ stages:
     - opam install -y -v -j ${NJOBS} coq-prosa
 
 .preferred-stable-version:
-    image: mathcomp/mathcomp:1.15.0-coq-8.15
+    image: mathcomp/mathcomp:1.15.0-coq-8.16
 
 .build:
   image: mathcomp/mathcomp:${CI_JOB_NAME}
@@ -70,11 +70,6 @@ stages:
     - .build
     - .not_in_wip_branches
 
-1.15.0-coq-8.16:
-  extends:
-    - .build
-    - .not_in_wip_branches
-
 compile-and-doc:
   stage: build
   extends: