ci(deps): bump docker/build-push-action from 4 to 6

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](https://github.com/docker/build-push-action/compare/v4...v6)

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

Signed-off-by: dependabot[bot] <support@github.com>

.github/workflows/release-slackbot-proxy.yml

@@ -51,7 +51,7 @@ jobs:
       uses: docker/setup-buildx-action@v3
 
     - name: Build and push
-      uses: docker/build-push-action@v4
+      uses: docker/build-push-action@v6
       with:
         context: .
         file: ./apps/slackbot-proxy/docker/Dockerfile