Update actions/cache action to v6

Author: renovate[bot]

.github/workflows/nextjs.yml

@@ -61,7 +61,7 @@ jobs:
           # You may remove this line if you want to manage the configuration yourself.
           static_site_generator: next
       - name: Restore cache
-        uses: actions/cache@v3
+        uses: actions/cache@v6
         with:
           path: |
             .next/cache