TinkerbellMachineTemplate.Spec is immutable during cluster upgrade

**What happened**:
When upgrading the cluster, EKSA throws error about "TinkerbellMachineTemplate.Spec is immutable"

**What you expected to happen**:
EKSA allows TinkerbellMachineTemplate.Spec in cluster upgrade.

**How to reproduce it (as minimally and precisely as possible)**:
Edit the TinkerbellMachineTemplate section, and trigger a cluster upgrade command:
```
eksctl anywhere upgrade cluster -f ./eksa-mgmt03-workers.yaml --kubeconfig ./mgmt03/mgmt03-eks-a-cluster.kubeconfig --no-timeouts -v 9
```

**Anything else we need to know?**:
Please refer to the following log:
```
\nversion: \\\"0.1\\\"\\n\"}}}}\nto:\nResource: \"infrastructure.cluster.x-k8s.io/v1beta1, Resource=tinkerbellmachinetemplates\", GroupVersionKind: \"infrastructure.cluster.x-k8s.io/v1beta1, Kind=TinkerbellMachineTemplate\"\nName: \"mgmt03-worker-01-control-plane-template-1699424535180\", Namespace: \"eksa-system\"\nfor: \"STDIN\": error when patching \"STDIN\": admission webhook \"validation.tinkerbellmachinetemplate.infrastructure.x-k8s.io\" denied the request: TinkerbellMachineTemplate.Spec is immutable\n", "retries": 29}
```

**Environment**:
- EKS Anywhere Release: 0.18.0
- EKS Distro Release: N/A

Start by reproducing the upgrade with the provided eksctl anywhere command and review the admission webhook error for TinkerbellMachineTemplate. Trace the cluster-upgrade handling of the edited TinkerbellMachineTemplate section; done means the upgrade no longer attempts an invalid Spec change and completes successfully.

Written by the indexing model from the issue text.