Upgrade checkout action to v4

Author: dinosaure

.github/workflows/main.yml

@@ -8,7 +8,7 @@ jobs:
         ocaml-version: ["5.3.0"]
     runs-on: ${{ matrix.operating-system }}
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v4
       with:
         lfs: true
     - uses: ocaml/setup-ocaml@v3