Update actions/cache to a newer version (v3)

As per
https://github.blog/changelog/2023-06-13-github-actions-all-actions-will-run-on-node16-instead-of-node12-by-default/,
GitHub has started a deprecation process for the GitHub Actions that run on
Node12. One of such actions was a `v2` of the `actions/cache` action. We're
updating the action to a newer `v3` version which runs on Node16.

.github/workflows/monitoring.yml

@@ -84,7 +84,7 @@ jobs:
       - uses: docker/setup-buildx-action@v2
 
       - name: Cache Docker layers
-        uses: actions/cache@v2
+        uses: actions/cache@v3
         with:
           path: /tmp/.buildx-cache
           key: ${{ runner.os }}-buildx-${{ github.sha }}