[github-actions]

• Deprecate airflow.utils.email in favor of the SMTP provider

The email utilities now live in airflow.providers.smtp.utils.emailer.
airflow.utils.email keeps its public names as deprecated aliases of the
provider functions, so existing imports and email_backend settings keep
working with a DeprecatedImportWarning.

The default [email] email_backend value is unchanged. The Task SDK treats it
as "no custom backend" and sends alerts with SmtpNotifier, as before.

A regression test covers email sent from a server process, such as a
listener in the API server, with the SMTP login stored only in the metadata
database.

airflow-core now requires apache-airflow-providers-smtp>=3.2.0, the first
release that contains the emailer module.

• Keep star imports of airflow.utils.email working and document the move

"from airflow.utils.email import *" imported the email functions before
they moved to the SMTP provider. Listing them in all keeps that import
working, with the same deprecation warning as named imports.

The email configuration guide now says where the functions live, that the
default email_backend value is unchanged, and that the moved functions do
not see settings changed at runtime with airflow.configuration.conf.set().

The coverage script entry for airflow/utils/email.py is left unchanged, so
this change stays within airflow-core.

• Shorten the core email page and point to the SMTP provider docs

The SMTP server and connection setup is documented by the SMTP provider, so the core page now only explains email_backend and the [email] options shared by all backends, and links to the provider docs.

• Explain what changes for callers of the deprecated email functions

Add a significant newsfragment, keep email_backend at its default in the docs note, and name the Connection lookup and configuration source in the deprecation warning.

• Test that a plain script does not see a database-only SMTP Connection

Outside task and server processes the Connection lookup only searches environment variables and secrets backends, so the email is sent without a login. The newsfragment describes this; the test now covers it.
(cherry picked from commit ad6ec79)

Co-authored-by: Shivam Rastogi 6463385+shivaam@users.noreply.github.com