chore: add deprecation notice to README (#745)

* Add deprecation notice to README

Agent-Logs-Url: https://github.com/Azure/terraform-azurerm-aks/sessions/12bb9b78-9483-4e23-bdc4-7818d4ccd99b

Co-authored-by: lonegunmanb <2233414+lonegunmanb@users.noreply.github.com>

* Remove Chinese text from deprecation notice, use English only

Agent-Logs-Url: https://github.com/Azure/terraform-azurerm-aks/sessions/3ab431eb-6332-4ca2-a0e0-88a2dc0092bd

Co-authored-by: lonegunmanb <2233414+lonegunmanb@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: lonegunmanb <2233414+lonegunmanb@users.noreply.github.com>

Author: Copilot

README.md

@@ -1,5 +1,13 @@
 # terraform-azurerm-aks
 
+> [!WARNING]
+> **⚠️ DEPRECATION NOTICE**
+>
+> This module is being retired. We will continue to fix bugs through April, but no new features will be accepted. The module is planned for official retirement in May.
+>
+> Please migrate to the new AVM module as soon as possible:
+> **[Azure/avm-res-containerservice-managedcluster/azurerm](https://registry.terraform.io/modules/Azure/avm-res-containerservice-managedcluster/azurerm/latest)**
+
 ## Deploys a Kubernetes cluster (AKS) on Azure with monitoring support through Azure Log Analytics
 
 This Terraform module deploys a Kubernetes cluster on Azure using AKS (Azure Kubernetes Service) and adds support for monitoring with Log Analytics.