Use checkout@v4 for docs build

Author: Joe Cummings

.github/workflows/build_docs.yaml

@@ -28,7 +28,7 @@ jobs:
         python-version: ['3.11']
     steps:
       - name: Check out repo
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
       - name: Setup conda env
         uses: conda-incubator/setup-miniconda@v2
         with: