Test older supported releases with newer Kubernetes versions

[agaudreault]

Summary

Argo CD documents the Kubernetes version that it has been tested on in https://argo-cd.readthedocs.io/en/stable/operator-manual/tested-kubernetes-versions/. This provides valuable information to cluster and Argo administrator to know if they can safely update their cluster or not.

When a new version of Argo CD is released, a new Kubernetes version will most likely have been released as well. The new version of Argo CD is tested against the new version of Kubernetes. However, the previous supported versions of Argo CD will never be tested against the new Kubernetes release, leaving cluster administrator in the dark on wether it is safe to upgrade the underlying Kubernetes cluster.

Motivation

Allow Argo administrator to safely know if they can update a Kubernetes cluster running a supported version of Argo CD.

Proposal

Current documentation

Argo CD version	Kubernetes versions
2.13	v1.30, v1.29, v1.28, v1.27
2.12	v1.29, v1.28, v1.27, v1.26
2.11	v1.28, v1.27, v1.26, v1.25

Expected documentation

Argo CD version	Tested Compatible Kubernetes versions
2.13	v1.30✅, v1.29✅, v1.28✅, v1.27✅
2.12	v1.30✅, v1.29✅, v1.28✅, v1.27✅, v1.26✅
2.11	v1.30❌, v1.29❌, v1.28✅, v1.27✅, v1.26✅, v1.25✅

In the documentation above, we retroactively updated the tested Kubernetes versions of the last 2 versions of Argo CD when 2.13 was released. If there was a change in Kubernetes 1.29 that caused Argo 2.11 to be incompatible, then it is now clearly identified in the latest documentation.

[rumstead]

We may need to think about how we handle the CI pipelines for cherry picks but overall I would find this useful.