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 They're not interchangeable — the TODO was wrong, not just outdated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