[9.2][Observability Onboarding] Add info about `helm` for the stateful use case in the quickstart guide for Elastic Agent on K8s

[vishaangelova]

With the 9.2 stack release, helm will replace kubectl kustomize in the Elastic Agent: Logs & Metrics quickstart workflow for Kubernetes data ingestion.

(Kustomize templates have been replaced by Helm charts in elastic/kibana#227037 and this change is already available on Serverless, and it will be live for the Stack in release 9.2)

Update the Quickstart: Monitor your Kubernetes cluster with Elastic Agent guide to reflect this change.

We’ll need to keep the current info with the kubectl kustomize command as it is relevant for 9.0 and 9.1. Perhaps we could add a separate tab for 9.2 - hopefully we’ll have applies_to support for tabs by the time this needs to be documented.

[vishaangelova]

The section related to Serverless where helm is already used in the quickstart install command is updated in #2433.

[eedugon]

@vishaangelova , when 9.2 comes and the UX quickstart workflow changes to helm, we will think of the best way to keep the doc valid for 9.0 & 9.1 releases too, but without putting too much emphasis on the kustomize method, as we are considering it deprecated in favor of the helm based installation.

Maybe just with a note explaining that for 9.0 and 9.1 the users will see kubectl kustomize being used instead of helm and recommending users to upgrade to a later version or even link users to the official docs to deploy the agent with helm without using the onboarding, so they can run the recommended method in 9.0 and 9.1.

Or, as you said, with tabs, but in the tab for 9.0 & 9.1 recommend the users to upgrade or use a helm based installation too.

The Elastic Agent installation through helm chart is fully supported in 9.x.