Bump actions/cache from 5 to 6

Bumps [actions/cache](https://github.com/actions/cache) from 5 to 6.
- [Release notes](https://github.com/actions/cache/releases)
- [Changelog](https://github.com/actions/cache/blob/main/RELEASES.md)
- [Commits](https://github.com/actions/cache/compare/v5...v6)

---
updated-dependencies:
- dependency-name: actions/cache
  dependency-version: '6'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

.github/workflows/check_links.yml

@@ -33,7 +33,7 @@ jobs:
       # Restore cache
       - name: Restore lychee cache
         id: restore-cache
-        uses: actions/cache/restore@v5
+        uses: actions/cache/restore@v6
         with:
           path: .lycheecache
           key: cache-lychee-${{ github.sha }}
@@ -97,7 +97,7 @@ jobs:
 
       # Always save cache
       - name: Save lychee cache
-        uses: actions/cache/save@v5
+        uses: actions/cache/save@v6
         if: always()
         with:
             path: .lycheecache