Fix failing backport packages test (#13497)

In #13473 - I updated the deprecated packages but looks like it broke backport packages:

```
    File "/usr/local/lib/python3.6/site-packages/airflow/providers/google/cloud/example_dags/example_tasks.py", line 32, in <module>
      from airflow.models.baseoperator import chain
  ImportError: cannot import name 'chain'
```

airflow/providers/google/cloud/example_dags/example_tasks.py

@@ -29,7 +29,6 @@
 from google.protobuf import timestamp_pb2
 
 from airflow import models
-from airflow.models.baseoperator import chain
 from airflow.operators.bash import BashOperator
 from airflow.providers.google.cloud.operators.tasks import (
     CloudTasksQueueCreateOperator,
@@ -47,6 +46,7 @@
     CloudTasksTasksListOperator,
 )
 from airflow.utils.dates import days_ago
+from airflow.utils.helpers import chain
 
 timestamp = timestamp_pb2.Timestamp()
 timestamp.FromDatetime(datetime.now() + timedelta(hours=12))  # pylint: disable=no-member