feat(cert-manager): upgrade to v1.20.2 and enable Gateway API support (#150)

## Summary

- Upgrades cert-manager from v1.19.2 to v1.20.2
- Enables `enableGatewayAPI: true` via the `ControllerConfiguration` config block

## Why

cert-manager's Gateway API integration was not enabled. Without it, `cert-manager.io/*` annotations on Gateway resources are ignored and no certificates are issued. This is required for the consul and vault PRs (#148, #149) to have their TLS certs automatically provisioned from their Gateway annotations.

In v1.20.2, `ExperimentalGatewayAPISupport` is BETA and defaults to true — enabling `enableGatewayAPI` in the controller config activates the gateway-shim controller.

## Test plan

- [ ] cert-manager rolls out cleanly (v1.20.2 pods become Ready)
- [ ] After rollout, existing Gateway-annotated services (artifactapi, puppet, litellm) retain valid certs
- [ ] New Gateway resources with `cert-manager.io/cluster-issuer` annotations trigger Certificate creation

Reviewed-on: #150

apps/overlays/au-syd1/cert-manager/kustomization.yaml

@@ -8,7 +8,7 @@ resources:
 helmCharts:
   - name: cert-manager
     repo: https://artifactapi.k8s.syd1.au.unkin.net/api/v1/virtual/helm
-    version: "v1.19.2"
+    version: "v1.20.2"
     releaseName: cert-manager
     namespace: cert-manager
     valuesFile: values.yaml