Use `--server-side` apply for HTTPRoute in conformance tests

The experimental channel conformance tests from the main branch are failing:

```
The CustomResourceDefinition "httproutes.gateway.networking.k8s.io" is invalid: metadata.annotations: Too long: must have at most 262144 bytes
```

The solution is to use `--server-side` apply for the Gateway API CRDs

See https://github.com/nginx/nginx-gateway-fabric/actions/runs/17256977734/job/48971158289