bump GH actions version to avoid node version warning in CI

Author: sjpb

.github/workflows/ci.yml

@@ -47,7 +47,7 @@ jobs:
 
     steps:
       - name: Check out the codebase.
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
 
       - name: Set up Python 3.
         uses: actions/setup-python@v4