Unable to set worker version equal to control plane version in 1.20.x

[Wesley Bermbach]

How to categorize this issue?

/kind bug
/platform azure

What happened:

It is not possible to set the Kubernetes version of a worker for a Kubernetes cluster in version 1.20.x to the same version as the control plane.

For example when trying to set the worker version to 1.20.15 for a cluster that is version 1.20.15 we get the following error.

admission webhook "validation.azure.provider.extensions.gardener.cloud" denied the request: spec.provider.workers[0].kubernetes.version: Forbidden: cannot use kubelet version (1.20.15) lower than CSI migration version (1.21.0)

What you expected to happen:

Set workers.kubernetes.version

How to reproduce it (as minimally and precisely as possible):

Create a cluster in version 1.20.x.
Set the worker kubernetes version equal to the control plane version.

[Konstantinos Angelopoulos]

This is by design to prevent cases where the control plane is migrated to CSI and the kubelet is running in a version that doesn't support it as indicated by #429 (which is the PR that adds the check you are referring to)

Maybe @rfranzke can add more, but I think that this is not a bug.