Clean up legacy SQLAlchemy references and update remaining doc links

[vishalmore90]

Context

PR #69200 updated several SQLAlchemy documentation links from en/14 (SQLAlchemy 1.4) to en/20 (SQLAlchemy 2.0), reflecting that Airflow core now requires SQLAlchemy 2.0+.
However, that PR:

1. Missed updating the pooling documentation link in config.yml.
2. Incorrectly updated a historical 1.4 changelog link in set-up-database.rst (where the postgres:// scheme deprecation was introduced) to point to the en/20 docs.
3. Left behind outdated warning prose referring to "SQLAlchemy 1.4+" in the database setup guide, which is no longer relevant now that Airflow 3 requires SQLAlchemy 2.0+ exclusively.

This PR addresses these omissions and the feedback from issue #69164.

Verification

• Run breeze build-docs (Docker environment not available locally, but the syntax, links, and anchors have been manually verified to resolve correctly).
• Verified target URLs:
  • Reverted link: https://docs.sqlalchemy.org/en/14/changelog/changelog_14.html#change-3687655465c25a39b968b4f5f6e9170b
  • Updated link: https://docs.sqlalchemy.org/en/20/core/pooling.html#disconnect-handling-pessimistic

Checklist

• Link to the issues/PRs: Closes Update SQLAlchemy documentation links from 1.4 (en/14) to 2.0 (en/20) #69164
• Documentation-only change (no newsfragment needed)

[boring-cyborg]

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[vishalmore90]

@vincbeck The SQLAlchemy 2.0 documentation (en/20) still hosts the SQLAlchemy 1.4 changelogs at /en/20/changelog/changelog_14.html.
Since Airflow core now exclusively requires SQLAlchemy 2.0+, it preferred to point all documentation links to the /en/20/ namespace so that users remain within the modern documentation layout and navigation context. I will revert this link to en/20.

[vincbeck]

Static checks are failing

[bugraoz93]

Overall looks good, I have comment on prek hook definition. PR is blocked because of log files pushed to PR, that is the breaking part we should address

[bugraoz93]

Why we need Python here as Python defined as language. Do we have exception for this hook not working without it similar to others?

[bugraoz93]

Same here and I think for all hooks :)

[bugraoz93]

This should be deleted

[bugraoz93]

This as well please. Not a good practice to push the logs maybe snippets in PR description would be good if these wanted to be shared

[vishalmore90]

Static checks are failing