Fix remaining SQLAlchemy 1.4/2.1 doc references and fact-check migration

[joshuabvarghese]

Addresses #69164

(follow-up to #69203 )

What this does

#69200 already fixed most of the 1.4→2.0 doc links. This PR:

1. Fixes the one remaining en/14 link (config.yml, sql_alchemy_pool_pre_ping)
2. Fixes an adjacent en/21 (beta) link that wasn't in the original 9 but has
   the same root problem — airflow-core requires sqlalchemy>=2.0.48, not 2.1
3. Does the fact-check the issue actually asked for (not just link-swapping)

Verification table

Location	Old	New	How verified
config.yml:686	en/14#disconnect-handling-pessimistic	en/20#disconnect-handling-pessimistic	Fetched live page, confirmed the anchor exists; confirmed our description text still matches 2.0 behavior
test_orm_event_handlers.py:20	en/21 (beta)	en/20	Confirmed identical content/anchor exists on en/20; consistent with the rest of the codebase and our actual version floor
utils/db.py:93-94	Stale TODO suggesting a swap to sqlalchemy.orm.MappedClassProtocol	Corrected comment, TODO removed	The real class checks __table__/__mapper__/__call__; ours checks __tablename__. They're not interchangeable — the TODO was wrong, not just outdated

Deliberately NOT changed

• set-up-database.rst:201 — correctly stays on en/20/changelog/changelog_14.html even
  though it documents 1.4-era behavior, because SQLAlchemy hosts historical changelogs
  under the current docs version's URL tree. settings.py already relies on this same
  pattern independently, which cross-confirms it's correct.
• Historical "as of SQLAlchemy 1.4..." mentions in configuration.py, provider changelogs,
  and RELEASE_NOTES.rst — true historical statements, not stale references.

Testing

• pre-commit run --files airflow-core/src/airflow/config_templates/config.yml airflow-core/src/airflow/utils/db.py airflow-core/tests/unit/utils/test_orm_event_handlers.py — all content-inspecting hooks pass (ruff, mypy, yamllint, license headers, codespell, newsfragment check)
• Manually re-verified the changed links by clicking through myself

---

Important

🛠️ Maintainer triage note for @joshuabvarghese · by @potiuk · 2026-07-28 16:10 UTC

Helpful heads-up from the maintainers — please address before this PR can be reviewed:

• ❌ Provider tests. See docs.

The ball is in your court — you've been assigned to this PR. Fix the above, then mark it Ready for review.

See the Pull Request quality criteria for how to fix each item. There is no rush.

Note: your branch is 362 commits behind main — please rebase and push again to get up-to-date CI results.

_{Automated triage — may be imperfect; a maintainer takes the next look. We use this two-stage triage process so maintainers' limited time goes to the conversation with you.}

[github-actions]

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