Remote Cluster setup docs are outdated

[mmahacek]

The docs for configuring remote clusters, specifically for ECK at page https://www.elastic.co/docs/deploy-manage/remote-clusters/eck-remote-clusters have some issues that are confusing to users.

The page indicates that there are two security models, API key and TLS certificate. The TLS method is also documented as deprecated in v9+. There are steps on how to use the API key model when both clusters are in the same ECK environment, but in the section for connecting clusters outside of the same ECK environment, only TLS certificate instructions are provided. As the steps listed are only for the deprecated method, there are no valid steps for deploying remote clusters on v9.

This page needs an update to include the following:

• Steps for configuring remote clusters on different K8s environments using the API key method
• Notes on each of the TLS method sections reminding users that these steps only apply to v7/v8 deployments