chore: Bump actions/cache from 5 to 6 (#764)

Author: dependabot[bot]

.github/workflows/check-links.yml

@@ -25,7 +25,7 @@ jobs:
       # temporary (rate limiting, network issue, etc.), and we always want to retry those links
       # everytime this action is run.
       - name: Restore lychee cache
-        uses: actions/cache@v5
+        uses: actions/cache@v6
         with:
           path: .lycheecache
           key: cache-lychee-${{ github.sha }}