Google GKE system Dags use deprecated cluster fields

### Under which category would you file this issue? Providers ### Apache Airflow version `main` (development) ### What happened and how to reproduce it? This is a follow-up to #69822. Removing the Vertex AI Feature Store system Dag's expensive parse-time imports allowed the generic system Dag import and serialization checks to proceed far enough to expose a separate problem in five Google Kubernetes Engine system Dags. The GKE examples set `initial_node_count` directly on the cluster payload. The Google provider warns that this cluster-level field is deprecated and recommends configuring the initial node count on a node pool instead. Consequently, checks that treat provider deprecation warnings as errors fail while importing these example Dags. Steps to reproduce: 1. Check out `main` with the Google provider development dependencies installed. 2. Run the affected example Dag import tests in Breeze while treating the warning as an error: ```bash breeze run pytest airflow-core/tests/unit/always/test_example_dags.py \ -k 'example_kubernetes_engine' \ -W 'error:The body field.*is deprecated:UserWarning' \ -q --disable-warnings ``` 3. Observe that importing the GKE system Dags fails because their cluster payloads use the deprecated top-level `initial_node_count` field. The affected examples are: - `example_kubernetes_engine.py` - `example_kubernetes_engine_async.py` - `example_kubernetes_engine_job.py` - `example_kubernetes_engine_kueue.py` - `example_kubernetes_engine_resource.py` ### What you think should happen instead? The GKE system Dags should define an explicit node pool and set `initial_node_count` on that node pool. They should import and serialize without emitting the cluster-level field deprecation warning. ### Operating System _No response_ ### Deployment None ### Apache Airflow Provider(s) _No response_ ### Versions of Apache Airflow Providers _No response_ ### Official Helm Chart version Not Applicable ### Kubernetes Versio…