Decouple SMTP Provider settings from the Airflow Core

[Taragolis]

Body

By mistake we add coupling between SMTP provider and core in #36226.
Discussion for the reference: #37701 (comment)

Provider Changes

• Move configurations into the provider configurations and get this settings from there

Airflow Core Changes

• Deprecate Remove SMTP_DEFAULT_TEMPLATED_SUBJECT and SMTP_DEFAULT_TEMPLATED_HTML_CONTENT_PATH into the airflow/settings.py
  - E.g. on access raise a deprecation warning with suggestion to upgrade to the new version of the SMTP Provider
  - E.g. on set from airflow_local_settings.py raise a deprecation warning with suggestion to upgrade to the new version of the SMTP Provider and use configurations instead

Committer

• I acknowledge that I am a maintainer/committer of the Apache Airflow project.

[eladkal]

It should be part of #30530
@hussein-awala started working on it a while ago in
#30531

[Taragolis]

Not really. This one about remove unnecessary settings options from the SMPT Provider, which add accidentally into the SmtpNotifier and Airflow 2.8.1 my bad it would be add into Airflow 2.9, so it should be easier to remove it from core.

[Taragolis]

So there is no such attributes into the settings.py into the Airflow 2.8.1 and 2.8.2

• https://github.com/apache/airflow/blob/2.8.1/airflow/settings.py
• https://github.com/apache/airflow/blob/2.8.2/airflow/settings.py

So we might just fix it into the provider, and remove it from the core until we release 2.9

[vchiapaikeo]

Oh nice! If we can just remove those configs, that would make things a lot easier. I was thinking of swapping them over to provider configuration w/ something like this:

#37711

Wdyt? Probably need to fix up a few tests but need to run right now so can look a little closer tonight.

[potiuk]

We just discussed it in release management channel - we will just remove the settings altogether (and yes replace with config) - we will also yank 1.6.0 (likely) and release 1.7.0 and also switch constraints for Airfrlow 2.8.1 and 2.8.2 to use smtp 1.5.0

[potiuk]

You can join #release-management channel on Slack for that one @vchiapaikeo :)

[vchiapaikeo]

Awesome, sounds good. Sorry about all the hassle here. Will get to fixing up that PR tonight!

[potiuk]

Well. We did not do a good job on preventing it during review, so don't put all the blame on you :)