Upgrading an EKS-A cluster with the original cluster config without the SSH keys removes the SSH keys

**What happened**:
After running the upgrade command with the original cluster config, where the SSH keys are auto-generated, removes the SSH key from the cluster nodes.

Describing the `KubeadmControlPlane` shows that the SSH Authorized keys are empty
```
Users:
Name: ec2-user
Ssh Authorized Keys:

Sudo: ALL=(ALL) NOPASSWD:ALL
```

**What you expected to happen**:
Since the SSH keys were auto-generated during the `create cluster` command, it is expected that the same keys will be used for upgrades as well.
But if you run upgrade with the original cluster config, the SSH keys are removed from the cluster node
```yaml
apiVersion: anywhere.eks.amazonaws.com/v1alpha1
kind: VSphereMachineConfig
metadata:
name: abhinav-workload
spec:
datastore: "WorkloadDatastore"
diskGiB: 25
folder: "abhinav-workload"
memoryMiB: 8192
numCPUs: 2
osFamily: bottlerocket
resourcePool: "Compute-ResourcePool"
users:
- name: ec2-user
```

**How to reproduce it (as minimally and precisely as possible)**:
- Create a cluster without specifying the SSH keys
- EKS-A cli will generate an SSH key, and configure it on the cluster nodes
- Run the upgrade command using the same cluster config
- The auto-generated SSH key gets removed from the cluster nodes

**Anything else we need to know?**:

**Environment**:
- EKS Anywhere Release: v0.11.1
- EKS Distro Release: v1.23
- OS: Bottlerocket

Start by tracing the upgrade command's handling of the original cluster config, then inspect how KubeadmControlPlane and VSphereMachineConfig represent users and SSH authorized keys for Bottlerocket. Reproduce the v0.11.1 scenario by creating a cluster without SSH keys and upgrading with the same config. Done means the auto-generated key remains configured on the cluster nodes after upgrade.

Written by the indexing model from the issue text.