Remove email configs, operator and callbacks

[hussein-awala]

This PR:

• removes the ability to use email_on_retry and email_on_failure to notify the users by email in favor of the SMTP notifier
• removes EmailOperator from airflow.operators.email in favor of airflow.providers.smtp.operators.smtp.EmailOperator
• cleans their usage and remove SMTP section from Airflow configuration

closes: #30530

[hussein-awala]

      - id: check-no-providers-in-core-examples
        language: pygrep
        name: No providers imports in core example DAGs
        description: The core example DAGs have no dependencies other than standard provider or core Airflow
        entry: "^\\s*from airflow\\.providers.(?!standard.)"
        pass_filenames: true
        files: ^airflow/example_dags/.*\.py$

What about pre-installed providers? if we really want to keep the rule as it is, then I will need to update the example.

[eladkal]

I don't think we should do that. We should keep pre-installed to the minimum necessary.
Probably raise this as one of the items in the lazy consensus

[eladkal]

We need also to remove the email and smtp part of airflow.cfg
https://airflow.apache.org/docs/apache-airflow/stable/configurations-ref.html#email

https://airflow.apache.org/docs/apache-airflow/stable/configurations-ref.html#smtp