[Onboarding] Update the Quickstart doc for monitoring of Kubernetes.

[akhileshpok]

The 'Quickstart: Monitor your Kubernetes cluster with Elastic Agent' doc should be updated.

• This functionality is available as GA in 8.16.
• This functionality is available in all Elastic deployment models.

Please reach out to @akhileshpok for clarifications on any of the above items.