CI: test mathcomp 1.18

Author: vbgl

.github/workflows/ci.yml

@@ -31,6 +31,9 @@ jobs:
         -  mathcomp/mathcomp:1.17.0-coq-8.16
         -  mathcomp/mathcomp:1.17.0-coq-8.17
         -  mathcomp/mathcomp:1.17.0-coq-8.18
+        -  mathcomp/mathcomp:1.18.0-coq-8.16
+        -  mathcomp/mathcomp:1.18.0-coq-8.17
+        -  mathcomp/mathcomp:1.18.0-coq-8.18
       fail-fast: false
     steps:
     - uses: actions/checkout@v3