chore(deps): update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/ci.yml

@@ -6,7 +6,7 @@ jobs:
   check-links:
     runs-on: ubuntu-24.04
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
     - name: Lychee Broken Link Checker
       uses: lycheeverse/lychee-action@v2.7.0
       with: