fix: update workflow dependencies

- Upgrade actions/checkout from v4 to v6
- Upgrade Python from 3.11 to 3.13

.github/workflows/sync-boostlook-css.yml

@@ -16,10 +16,10 @@ jobs:
     if: github.repository == 'boostorg/boostlook'
     steps:
       - name: Checkout current repository
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
 
       - name: Checkout website-v2 repository
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
         with:
           repository: boostorg/website-v2
           ref: develop
@@ -29,7 +29,7 @@ jobs:
       - name: Set up Python
         uses: actions/setup-python@v4
         with:
-          python-version: "3.11"
+          python-version: "3.13"
 
       - name: Copy boostlook.css to website-v2
         run: |