docs: redirect Azure Stack Hub users to new repository (#5073)

* docs: migration note for Azure Stack Hub users

* more context

Author: jadarsie

Diff for: .github/ISSUE_TEMPLATE/azure_stack_hub.md

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+---
+name: Azure Stack Hub issue or feature
+about: Migration note for Azure Stack Hub users
+title: ''
+labels: ''
+assignees: ''
+
+---
+
+As this project is deprecated for Azure public cloud customers, development has been moved to a new Azure Stack Hub specific repository ([Azure/aks-engine-azurestack](https://github.com/Azure/aks-engine-azurestack)). This new repository is where new [releases](https://github.com/Azure/aks-engine-azurestack/releases) for Azure Stack Hub clouds, starting at v0.75.3, will be published and where [issues](https://github.com/Azure/aks-engine-azurestack/issues/new) concerning Azure Stack Hub should be created.

Diff for: README.md

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 This project is deprecated for Azure public cloud customers. Please consider using [Azure Kubernetes Service (AKS)](https://azure.microsoft.com/en-us/services/kubernetes-service/#overview) for managed Kubernetes or [Cluster API Provider Azure](https://github.com/kubernetes-sigs/cluster-api-provider-azure) for self-managed Kubernetes. There are no new features planned; this project will only be updated for CVEs & similar, with Kubernetes 1.24 as the final version to receive updates.
 
-For use on [Azure Stack Hub product](https://docs.microsoft.com/en-us/azure-stack/user/azure-stack-kubernetes-aks-engine-overview) this project is fully supported and will continue to be supported by the Hub team throughout the lifespan of Azure Stack Hub. 
+For use on the [Azure Stack Hub product](https://docs.microsoft.com/en-us/azure-stack/user/azure-stack-kubernetes-aks-engine-overview) this project is fully supported and will continue to be supported by the Hub team throughout the lifespan of Azure Stack Hub. Development is already moved to a new Azure Stack Hub specific repository ([Azure/aks-engine-azurestack](https://github.com/Azure/aks-engine-azurestack)). This new repository is where new [releases](https://github.com/Azure/aks-engine-azurestack/releases) for Azure Stack Hub clouds, starting at v0.75.3, will be published and where [issues](https://github.com/Azure/aks-engine-azurestack/issues/new) concerning Azure Stack Hub should be created.
 
 ## Overview
 

Diff for: docs/topics/azure-stack.md

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 # AKS Engine on Azure Stack Hub
 
+> **Migration note**: As this project is deprecated for Azure public cloud customers, development has been moved to a new Azure Stack Hub specific repository ([Azure/aks-engine-azurestack](https://github.com/Azure/aks-engine-azurestack)). This new repository is where new [releases](https://github.com/Azure/aks-engine-azurestack/releases) for Azure Stack Hub clouds, starting at v0.75.3, will be published and where [issues](https://github.com/Azure/aks-engine-azurestack/issues/new) concerning Azure Stack Hub should be created.
+
 * [Introduction](#introduction)
 * [Marketplace Prerequisites](#marketplace-prerequisites)
 * [Service Principals and Identity Providers](#service-principals-and-identity-providers)