Update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/format_rst.yml

@@ -7,7 +7,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Check out code
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
 
       - name: Set up Python
         uses: actions/setup-python@v5