update setup-ocaml version in CI

Author: Firobe

.github/workflows/ci.yml

@@ -23,7 +23,7 @@ jobs:
         uses: actions/checkout@v2
 
       - name: Use OCaml ${{ matrix.ocaml-compiler }}
-        uses: ocaml/setup-ocaml@v2
+        uses: ocaml/setup-ocaml@v3
         with:
           ocaml-compiler: ${{ matrix.ocaml-compiler }}