Update Remote clusters setup between ECH and ECK to use API Key authentication

[eedugon]

The following doc describes how to setup remote clusters between ECH and ECK: https://www.elastic.co/docs/deploy-manage/remote-clusters/ec-enable-ccs-for-eck

I'd say the docs is still perfectly valid, but it's using TLS cert based authentication for remote clusters configuration, a deprecated method that might be removed in the future. When possible we are recommending users to move to API based authentication.

As soon as API key authentication is feasible for this use case we should update our documentation.

I'm not sure if this is feasible at the moment (cc: @elastic/cloud-k8s & @elastic/control-plane-applications ).

It would be great if we can get some feedback from cloud PMs (@Kushmaro perhaps).

[pebrc]

@eedugon please see this this issue with instructions from Michael Morello elastic/cloud-on-k8s#8502

[eedugon]

@shainaraskas : We now have 3 different issues all related with remote clusters configuration on ECK (mainly to document API key method). It would be great if we find the time to prioritize this in the near future, maybe with some extra refinement of remote clusters section in general (if needed).

I've linked the other 2 issues as parent issues.