Update actions/cache action to v6

Author: renovate[bot]

.github/workflows/ci.yml

@@ -41,7 +41,7 @@ jobs:
         uses: actions/checkout@v3.5.2
 
       - name: Restore APT cache
-        uses: actions/cache@v3
+        uses: actions/cache@v6
         with:
           path: /var/cache/apt
           key: ${{ runner.os }}-apt