Update actions/checkout action to v5

Author: renovate[bot]

.github/workflows/mkdocs.yaml

@@ -10,7 +10,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout master
-        uses: actions/checkout@v4
+        uses: actions/checkout@v5
 
       - name: Deploy docs
         uses: mhausenblas/mkdocs-deploy-gh-pages@master