Remove reference to klar for container scanning

What does this Merge Request do and why?

This MR updates the docs related to container scanning:

• klar is deprecated and is replaced by gcs which uses Trivy or Grype as the scanners under the hood based on the given argument.
• Update REGISTRY_INSECURE and DOCKER_INSECURE to CS_REGISTRY_INSECURE and CS_DOCKER_INSECURE

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.