Update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/documentation.yaml

@@ -17,7 +17,7 @@ jobs:
       name: Setup TFLint
       with:
         tflint_version: v0.38.1
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v6
     - uses: actions/setup-python@v4
     - uses: pre-commit/[email protected]
       # pre-commit fails if it changed files