Move all application logs in the Elasticsearch GKE index to other indexes

The GKE index should be reserved for system logs, e.g. kubelet, kube-proxy.

Application logs (such as the Docker registry) should be in their own indexes, as we have always done outside of GKE, and as is common practice for logging use cases of Elasticsearch.

Examine where the logs in this index are coming from (probably by looking at what ends up in the GKE pubsub topic, via stackdriver sinks / fluentd config), and redirect application logs to their own indexes. The implementation of this will likely interact with https://gitlab.com/gitlab-com/gl-infra/infrastructure/issues/9120.