Bump docker/build-push-action from 4 to 6 (#6390)

Bumps [docker/build-push-action](https://github.com/docker/build-push-action) from 4 to 6.
- [Release notes](https://github.com/docker/build-push-action/releases)
- [Commits](docker/build-push-action@v4...v6)

---
updated-dependencies:
- dependency-name: docker/build-push-action
  dependency-version: '6'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

.github/workflows/knative-docs-image.yaml

@@ -56,15 +56,15 @@ jobs:
           tags: ghcr.io/knative/autoscale-go:latest
 
       - name: Build sample-app's frontend image
-        uses: docker/build-push-action@v4
+        uses: docker/build-push-action@v6
         with:
           push: true
           platforms: linux/amd64,linux/arm64
           context: ./code-samples/eventing/bookstore-sample-app/solution/frontend
           tags: ghcr.io/knative/bookstore-frontend:latest
 
       - name: Build sample-app's node-server image
-        uses: docker/build-push-action@v4
+        uses: docker/build-push-action@v6
         with:
           push: true
           platforms: linux/amd64,linux/arm64