Bump setup-ocaml version

Author: liam923

.github/workflows/flambda-backend.yml

@@ -44,7 +44,7 @@ jobs:
         ocaml-compiler: ${{ matrix.ocaml-compiler }}
 
     - name: Cache flambda-backend build
-      uses: actions/cache@v2
+      uses: actions/cache@v3
       id: cache
       with:
         path: ${{ github.workspace }}/flambda-backend/_install