Unable to remove useExperimentalRetryJoin field from KubeadmControlPlaneTemplate

[MaxFedotov]

What steps did you take and what happened?

UseExperimentalRetryJoin param in KubeadmConfigSpec is marked as deprecated and is planned to be removed in next releases.

As a part of clusterClass refactoring, I removed useExperimentalRetryJoin field from my KubeadmControlPlaneTemplate for a new clusterClass version.

After updating cluster to use a new clusterClass the following error occurred:

E0811 14:01:32.036524       1 controller.go:329] "Reconciler error" err="error reconciling the Cluster topology: failed to create patch helper for KubeadmControlPlane/gcore-eu-mf-test1-10-bn6kn: server side apply dry-run failed for modified object: admission webhook \"validation.kubeadmcontrolplane.controlplane.cluster.x-k8s.io\" denied the request: KubeadmControlPlane.controlplane.cluster.x-k8s.io \"gcore-eu-mf-test1-10-bn6kn\" is invalid: spec.kubeadmConfigSpec.useExperimentalRetryJoin: Forbidden: cannot be modified" controller="topology/cluster" controllerGroup="cluster.x-k8s.io" controllerKind="Cluster" Cluster="system-clusters/gcore-eu-mf-test1-10" namespace="system-clusters" name="gcore-eu-mf-test1-10" reconcileID=6e10c68c-345b-41ff-861c-5d29add97b6a

What did you expect to happen?

CAPI should allow to drop deprecated useExperimentalRetryJoin from KubeadmConfigTemplate template.

Cluster API version

v1.4.1

Kubernetes version

v1.24.15

Anything else you would like to add?

No response

Label(s) to be applied

/kind bug
/area /provider/control-plane-kubeadm

[fabriziopandini]

/triage accepted

Historical context. In KCP we are incrementally enabling changes to the spec only after checking those changes are not leading to non-functional clusters.
This change seems legit and supportive of the deprecation notice, but in other cases, it will be correct that KCP denies the change acting as a safeguard for the cluster.