Make `@task` import from `airflow.sdk`

(follow-up of https://github.com/apache/airflow/pull/48683)

We should get the decorators in `https://github.com/apache/airflow/blob/main/airflow-core/src/airflow/decorators/__init__.py`  importable from `airflow.sdk` instead.

Especially `from airflow.sdk import task` should work.
