Use setup-ocaml 3 for opam workflow

Author: jmid

.github/workflows/opam.yml

@@ -34,7 +34,7 @@ jobs:
         uses: actions/checkout@v4
 
       - name: Install OCaml compiler
-        uses: ocaml/setup-ocaml@v2
+        uses: ocaml/setup-ocaml@v3
         with:
           ocaml-compiler: ${{ matrix.ocaml-compiler }}
           opam-depext: false