remove deprecated email utils and email_backend configs

[wjddn279]

With this PR(#57354), send_email from airflow.utils is no longer used in the task runner. Additionally, since email_backend is also no longer used, it has been marked as deprecated.

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Important

🛠️ Maintainer triage note for @wjddn279 · by @potiuk · 2026-06-18 13:57 UTC

Paused pending your next update — this PR has been inactive for ~35 days, so it's been moved to draft to keep the review queue clear:

• Rebase on the latest main, address any new failures, and mark it Ready for review when you pick it back up — no rush.
• See the Pull Request quality criteria.

The ball is in your court — you've been assigned to this PR.

_{Automated triage — may be imperfect; a maintainer takes the next look.}

[wjddn279]

pending until #65012 (comment)

[amoghrajesh]

send_email in airflow.utils.email is a public interface — removing it directly is a breaking change. It needs a deprecation cycle with an alternative before it can be deleted.

[wjddn279]

@amoghrajesh

Could you see this once again??