Retirement of Managed NGINX in application routing add-on

### Description

The Kubernetes SIG Network and Security Response Committee has publicly announced the upcoming retirement of the **Ingress-NGINX** controller. According to the official Kubernetes blog, best-effort maintenance will continue until March 2026, after which the project will no longer receive releases, bug fixes, or security updates. This marks the end-of-life for one of the most widely deployed community ingress controllers in Kubernetes.
[Link to upstream announcement](https://kubernetes.io/blog/2025/11/11/ingress-nginx-retirement)

For Azure Kubernetes Service (AKS), this retirement impacts clusters using Managed NGINX ingress via the Application Routing add-on. Microsoft has published an update outlining how AKS will support Application Routing with ingress-nginx through November 2026 for critical security patching and the ongoing evolution toward Gateway API and other alternatives.
[Link to AKS blog](https://blog.aks.azure.com/2025/11/13/ingress-nginx-update?utm_source=chatgpt.com)

This issue will track the retirement of Managed NGINX ingress in AKS, customer impact, timeline alignment with upstream Kubernetes, and coordinated communications. For the related feature effort to add Gateway API support in the Application Routing add-on, see: #5515