Adding or removing external etcd during an upgrade.

**What would you like to be added**:
Adding or removing external etcd during an upgrade.

**Why is this needed**:
I have a need to reduce the footprint of some EKS-A clusters by bringing etcd back onto the control plane nodes, reducing VMware footprint.

At present the upgrade validations fail with "adding or removing external etcd during upgrade is not supported".

Start by locating the upgrade validation that emits "adding or removing external etcd during upgrade is not supported." Trace the external-etcd and control-plane upgrade paths, then identify the validation and upgrade tests covering both adding and removing external etcd. Done means supported upgrades no longer fail this validation and the relevant tests pass.

Written by the indexing model from the issue text.