Add OCaml 5.1 to CI

Author: sim642

.github/workflows/ci.yml

@@ -15,7 +15,8 @@ jobs:
           - ubuntu-latest
           # - macos-latest
         ocaml-compiler:
-          - 5.0.0
+          - 5.1.x
+          - 5.0.x
           - 4.14.x
           - 4.13.x
           - 4.12.x