Description of problem

controlPlane.replicas install-config docs have:

Supported values are 3, or 1 when deploying single-node OpenShift

But while Agent recently backported some validation for that to 4.14.z (OCPBUGS-31885), I don't see anything in validateControlPlane enforcing the 1-or-3 requirement.

How reproducible

Likely 100%, although I haven't tested.

Steps to Reproduce

Use a non-Agent installer to install a cluster with an install-config setting controlPlane.replicas: 2.

Actual results

Cluster installation attempted.

Expected results

Installer rejects the invalid install-config before touching cloud resources.