removing deprecation postgres

[Bowrna]

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Related: https://github.com/apache/airflow/issues/39485 Removing deprecated PostgresOperator from docs and tests and replacing with SQLExecuteQueryOperator

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[Taragolis]

Looks really nice!

I would suggest to keep information about PostgresOperator into the documentation, e.g. in warning block This might help users who try to search PostgresOperator via search engines (e.g. in google). In this case, potentially, it come to the current version documentation page, otherwise it might show old version of provider.

Something like that

.. warning::
    Previously, PostgresOperator was used to perform this kind of operation. But at the moment PostgresOperator is deprecated and will be removed in future versions of the provider. Please consider to switch to SQLExecuteQueryOperator as soon as possible.

[Bowrna]

@Taragolis I have added the warning part in docs. Let me know if it looks good now. Will do the same for the other DB related providers too!