[Feature] AKS Automatic: Azure Container Linux as the default node image type

As cluster operator, I want [Azure Container Linux (ACL)](https://learn.microsoft.com/azure/azure-linux/azure-container-linux-overview) to be the default node image type so that my clusters run the most secure, cloud-optimized, and operationally resilient host OS without requiring additional configuration or decision-making.

[AKS Automatic](https://learn.microsoft.com/azure/aks/intro-aks-automatic) is designed around opinionated defaults that reduce operational burden and improve security outcomes, making ACL the best match for cluster nodes.