Add warning about the PostgresOperator being deprecated (apache#40662)

PostgresOperator's reference page says it's deprecated. I am not knowledgeable enough to write a tutorial. I think if someone reads the guide (I linked to in my change) they will be able to use this tutorial as well.

docs/apache-airflow/tutorial/pipeline.rst

@@ -23,6 +23,10 @@ Building a Running Pipeline
 
 Lets look at another example: we need to get some data from a file which is hosted online and insert it into our local database. We also need to look at removing duplicate rows while inserting.
 
+*Be advised:* The operator used in this tutorial is `deprecated <https://airflow.apache.org/docs/apache-airflow-providers-postgres/stable/_api/airflow/providers/postgres/operators/postgres/index.html>`_.
+Its recommended successor, `SQLExecuteQueryOperator <https://airflow.apache.org/docs/apache-airflow-providers-common-sql/stable/_api/airflow/providers/common/sql/operators/sql/index.html#airflow.providers.common.sql.operators.sql.SQLExecuteQueryOperator>`_ works similarly.
+You might find `this guide <https://airflow.apache.org/docs/apache-airflow-providers-postgres/stable/operators/postgres_operator_howto_guide.html#creating-a-postgres-database-table>`_ helpful.
+
 Initial setup
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