fix: align OpenShift registry defaults and docs

[vavarshn]

Summary

• Change the OpenShift images.registryInsecure default from false to true in the sample configuration, installer configuration loading, and air-gap image import.
• Preserve an explicitly configured images.registryInsecure: false for ECR, Docker Hub, Harbor, and other trusted HTTPS registries.
• Skip the insecure-registry policy preflight when no image registry is configured.
• Add test cases for the omitted default, explicit false, and empty-registry behavior.
• Update the OpenShift README, quick reference, and troubleshooting guide to describe the new registry behavior and ECR credential-refresh responsibility.
• Update the qualified installer and OpenShift node compatibility matrices with the supported installer versions and RHCOS 9.6.20260407-0 (Plow) on amd64.
• Update Splunk AI Assistant requirements to version 2.3.2 or later and document the onboarding wizard steps, including how to retrieve and enter the Splunk AI Assistant Route.
• Remove the k0s README reference from the OpenShift README.

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-04T12:43:48.025539Z	d0ed13f	PR opened

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