Add warning about the PostgresOperator being deprecated

[a1ea321]

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.

I used a direct URL to link to the guide. The :doc: type of linking did not render properly in the "Preview" tab of GitHub's editor.

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[boring-cyborg]

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