[Security] Improve secureness of dind root containers with "User Namespaces" feature.

What would you like added?

Because the DinD (Docker-in-Docker) pod runs as root, it should leverage Kubernetes “User Namespaces” for better isolation and security (see https://kubernetes.io/docs/tasks/configure-pod-container/user-namespaces/).

Enabling this feature only requires adding the field hostUsers: false to the pod template, for example here:
https://github.com/actions/actions-runner-controller/blob/master/charts/gha-runner-scale-set/templates/autoscalingrunnerset.yaml#L157

Why is this needed?

User Namespaces are a new Kubernetes security feature that significantly reduces the risk of privilege escalation. Security is a must-have. 😊

Additional context

• The spec.hostUsers field is available starting with Kubernetes 1.30 (to be confirm)
• Setting hostUsers: false activates User Namespaces only if clusters support them; otherwise clusters the field is simply ignored, so compatibility is maintained.

Start with charts/gha-runner-scale-set/templates/autoscalingrunnerset.yaml around the pod template at line 157, and review the Kubernetes User Namespaces documentation linked in the issue. Add the requested hostUsers setting and verify that the resulting template preserves compatibility for clusters that do not support the feature.