Bump actions/cache from 5 to 6 (#533)

Author: dependabot[bot]

.github/workflows/check_links.yml

@@ -13,7 +13,7 @@ jobs:
       - uses: actions/checkout@v7
       - name: Restore lychee cache
         id: restore-cache
-        uses: actions/cache/restore@v5
+        uses: actions/cache/restore@v6
         with:
           path: .lycheecache
           key: cache-lychee-${{ github.run_id }}-${{ github.run_attempt }}
@@ -40,7 +40,7 @@ jobs:
             --max-concurrency 1
             './**/*.md' './**/*.html' './**/*.rst' './**/*.cpp' './**/*.h' './**/*.py'
       - name: Save lychee cache
-        uses: actions/cache/save@v5
+        uses: actions/cache/save@v6
         if: always()
         with:
           path: .lycheecache