Updates references on how to reach the KAS ingress

What does this MR do?

After gitlab-org/charts/gitlab!1623 (merged) got merged, the path /kubernetes-agent is not anymore available in our KAS Ingress for GitLab Chart. The service will be available as a subdomain instead. I.e., if the domains is example.com, KAS will be available at kas.example.com.

For Omnibus, we're not updating the docs yet since the same is still work in progress: omnibus-gitlab!4724 (closed)

Related issues

gitlab-org/charts/gitlab#2373 (closed)

https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/11581

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