Kubernetes versions need to be updated in res-aka-cluster snippet


Currently supported is up to 1.21:
https://docs.microsoft.com/en-us/azure/aks/supported-kubernetes-versions?tabs=azure-cli

There are almost certainly other things (VM images for example) in the snippets that need updating. Maybe we need a process to manually or automatically update these every few months.

Locate the res-aka-cluster snippet and compare its Kubernetes version with the linked Azure AKS supported-versions page. Inspect the snippet for other potentially outdated items such as VM images, then update the documented values; done means the snippet reflects currently supported versions and any related stale values are addressed.

Written by the indexing model from the issue text.