Update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/docs_validate.yml

@@ -13,7 +13,7 @@ jobs:
     name: 📚 Doc validation
     runs-on: ubuntu-24.04
     steps:
-    - uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v5
+    - uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6
       with:
         fetch-depth: 0 # avoid shallow clone so nbgv can do its work.
     - name: 🔗 Markup Link Checker (mlc)