Refine and detail k8s cluster upgrade policy

docs/howto/upgrade-cluster/index.md

@@ -11,19 +11,26 @@ clusters on AWS.
 
 ## Upgrade policy
 
-We aim to ensure we use a k8s version for the control plane and node groups that is at least **five minor versions** behind the latest one available at any given time.
-
-Ideally, the following rules should also be respected:
-
-1. Every new cluster we deploy should be using the latest available kubernetes version.
-1. All of the clusters deployed in a cloud provider should be using the same version.
-1. Check if new upgrades are needed at least every 3 months.
-
-
-```{warning}
-As of now, we have not yet established practices on how to ensure these upgrades happen according to the policy above. Establishing this is tracked in [this GitHub
-issue](https://github.com/2i2c-org/infrastructure/issues/412).
-```
+1. To keep our k8s cluster's control plane and node pools upgraded to the latest
+   _three_ and _four_ [official minor k8s versions] respectively at all times.
+2. To await a level of maturity for minor k8s versions before we adopt them.
+
+   Kubernetes distribution | Our maturity criteria
+   -                       | -
+   GKE                     | Part of [GKE's regular release channel]
+   EKS                     | [Supported by `eksctl`] and GKE
+   AKS                     | Listed as [generally available on AKS]
+3. To upgrade k8s cluster's control plane and node pools at least _twice_ and
+   _once_ per year respectively.
+4. To not disrupt user nodes with running users, by instead rolling out new user
+   node pools if needed and cleaning up the old at a later time.
+5. To check if actions needs to be scheduled related to this in the beginning of
+   every quarter.
+
+[official minor k8s versions]: https://kubernetes.io/releases/
+[gke's regular release channel]: https://cloud.google.com/kubernetes-engine/docs/release-notes-regular
+[supported by `eksctl`]: https://eksctl.io/getting-started/#basic-cluster-creation
+[generally available on aks]: https://learn.microsoft.com/en-gb/azure/aks/supported-kubernetes-versions?tabs=azure-cli#aks-kubernetes-release-calendar
 
 ```{toctree}
 :maxdepth: 1