doc: Add a note about k8s version upgrades for AKS

doc/content/the-things-stack/host/kubernetes/azure/upgrading/_index.md

@@ -31,3 +31,7 @@ $ terraform apply -var='is-db-migrate=true' -var='noc-db-migrate=true'
 ```
 
 {{</ note >}}
+
+## Kubernetes version upgrade
+
+AKS Kubernetes version upgrades have to be performed sequentially for each minor version. For example to upgrade from `1.25` to `1.27` it's necessary to upgrade to version `1.26` first as an intermediate step. For more information refer to the [official documentation](https://learn.microsoft.com/en-us/azure/aks/supported-kubernetes-versions).