Fix CI by using setup-ocaml v3 for ocamlformat workflow (#3426)

[CI] Use setup-ocaml v3 for ocamlformat workflow

Author: rtjoa

.github/workflows/ocamlformat.yml

@@ -21,7 +21,7 @@ jobs:
         path: 'flambda_backend'
 
     - name: Setup OCaml ${{ matrix.ocaml-compiler }}
-      uses: ocaml/setup-ocaml@v2
+      uses: ocaml/setup-ocaml@v3
       with:
         ocaml-compiler: ${{ matrix.ocaml-compiler }}