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](actions/cache@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>

Author: dependabot[bot]

.github/workflows/link-checker-prs.yml

@@ -29,7 +29,7 @@ jobs:
           echo "MARKDOWN_FILES=$md_files" >> $GITHUB_ENV
 
       - name: Restore lychee cache
-        uses: actions/cache@v5
+        uses: actions/cache@v6
         with:
           path: .lycheecache
           key: cache-lychee-${{ github.sha }}

.github/workflows/link-checker.yml

@@ -18,7 +18,7 @@ jobs:
       - uses: actions/checkout@v7
 
       - name: Restore lychee cache
-        uses: actions/cache@v5
+        uses: actions/cache@v6
         with:
           path: .lycheecache
           key: cache-lychee-${{ github.sha }}