Update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/check-mkdocs-uris.yml

@@ -13,7 +13,7 @@ jobs:
 
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
 
       - name: Install dependencies
         run: |