[KRaft] Prevent mixed nodes changing to controller-only nodes if they still have assigned partition replicas

[scholzj]

When a user changes a node pool roles from mixed to controller-only without reassigning all partition replicas assigned to them, it might break the availability of these topics and block rolling updates. We should check this and prevent the change similarly to how we do it when scaling down broker nodes.

[scholzj]

Discussed in the community call on 14.12.: Should be implemented as a safeguard.