Deprecate smtp configs in airflow settings / local_settings

[vchiapaikeo]

closes: #37703

This removes the usage of airflow/settings.py | airflow_local_settings.py from the templated SmtpNotifier.

Ensure emails still come through as expected:

Using this test dag:

import datetime

from airflow import DAG
from airflow.operators.bash import BashOperator
from airflow.providers.smtp.notifications.smtp import SmtpNotifier
from datetime import timedelta


with DAG(
    dag_id="my_dag",
    start_date=datetime.datetime(2021, 1, 1),
    schedule="*/5 * * * *",
    catchup=False,
    max_active_tasks=1,
    max_active_runs=1,
    sla_miss_callback=SmtpNotifier(from_email=None, to="[email protected]"),
):
    BashOperator(
        task_id="task",
        bash_command="sleep 5 && exit 1",
        retries=1,
        sla=timedelta(seconds=30),
        on_success_callback=SmtpNotifier(from_email=None, to="[email protected]"),
        on_failure_callback=SmtpNotifier(from_email=None, to="[email protected]"),
        on_retry_callback=SmtpNotifier(from_email=None, to="[email protected]"),
    )

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[Taragolis]

Also would be nice to have a test with a custom values which comes thought configs.
Just in case, it could be done by a conf_vars context manager

[potiuk]

Also I think we should release 1.6.1 NOT 1.7.0. Technically it will be a bugfix to buggy feature added it 1.6.0. Even if we yank 1.6.0, this one is still adding the same feature already added in 1.6.* (only properly).

[potiuk]

Should be good to go @eladkal