Bump actions/cache from 5 to 6 (backport #1847) (#1852)

Author: mergify[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 }}
@@ -33,7 +33,7 @@ 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

.github/workflows/reusable_ici.yml

@@ -74,7 +74,7 @@ jobs:
           # into the URSim container.
           chmod 777 /tmp/ur_test_artifacts /tmp/ursim_urcaps
       - name: Cache ccache
-        uses: actions/cache@v5
+        uses: actions/cache@v6
         with:
           path: ${{ env.CCACHE_DIR }}
           key: ccache-${{ env.CACHE_PREFIX }}-${{ github.sha }}-${{ github.run_id }}