earlyRequestHeaders not accepted

Seeing the new feature with the `earlyRequestHeaders`, we found that it's essential for us so we upgraded to 1.2.1. We used `helm upgrade --install envoy-gateway oci://docker.io/envoyproxy/gateway-helm --version v1.2.1 -n envoy-gateway`

So now we have defined this resource, using syntax from the documentation page:
```
apiVersion: gateway.envoyproxy.io/v1alpha1
kind: ClientTrafficPolicy
metadata:
name: enable-early-headers
namespace: test
spec:
targetRef:
group: gateway.networking.k8s.io
kind: Gateway
name: apps
headers:
earlyRequestHeaders:
add:
- name: "x-header-erp"
value: "foobar"
```

...and when we apply it we get this error:

`ClientTrafficPolicy in version "v1alpha1" cannot be handled as a ClientTrafficPolicy: strict decoding error: unknown field "spec.headers.earlyRequestHeaders"`

What could we be missing here?