Update swarm intro with voting app

The [votingapp.md](https://github.com/docker/labs/blob/master/beginner/chapters/votingapp.md) file needs to be updated with the [latest stack file from example-voting-app](https://github.com/dockersamples/example-voting-app/blob/main/docker-stack.yml) and update/remove/cleanup some of the old info in that markdown.