Update ci to use ocaml/setup-ocaml@v3

Author: liam923

.github/workflows/flambda-backend.yml

@@ -38,7 +38,7 @@ jobs:
         path: 'merlin-jst'
 
     - name: Set up OCaml ${{ matrix.ocaml-compiler }}
-      uses: ocaml/setup-ocaml@v2
+      uses: ocaml/setup-ocaml@v3
       with:
         # Version of the OCaml compiler to initialise
         ocaml-compiler: ${{ matrix.ocaml-compiler }}