Update documentation to no longer suggest Classic Load Balancer

https://docs.gitlab.com/ee/install/aws/manual_install_aws.html#load-balancer

We currently suggest AWS Classic Load Balancer in our GitLab AWS POC. As we move towards more websocket implementation, we shouldn't suggest a load balancer that doesn't support websockets, as shown in Issue AWS Issue 10

Opening this issue for initial discussion into which load balancer might replace classic in our POC.