Clean up Podman with Kubernetes examples

What does this MR do?

Cleans up the Use Podman with GitLab Runner on Kubernetes doc based on user feedback.

Changes

• Removes feature flags from .gitlab-ci.yml examples and replaces them with a link to the feature flags page. FF_USE_ADVANCED_POD_SPEC_CONFIGURATION: "true" is kept in example 2 as it is required for the pod_spec configuration to work.
• Removes personal test data and internal fields from config.toml example 1: id, token, token_obtained_at, and user = "ratchade".
• Removes empty blocks from both config.toml examples: [[runners.kubernetes.services]], [runners.kubernetes.dns_config], and [runners.kubernetes.pod_labels].

Related issues

https://gitlab.com/gitlab-org/gitlab/-/work_items/595055

Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
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• ~"frontend"
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These labels cause the MR to be added to code verification QA issues.

Reviewer's checklist

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• If the content requires it, ensure the information is reviewed by a subject matter expert.
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