Update SQLAlchemy documentation links from 1.4 (en/14) to 2.0 (en/20)

## Human Summary

While reviewing #69150 (which fixes #25461), I noticed that a number of our
SQLAlchemy documentation links still point at the SQLAlchemy 1.4 docs (`en/14`), even though
`airflow-core` now requires SQLAlchemy **2.x** .

I don't usually open issues for documentation, but this one is better handled by
someone with SQLAlchemy domain knowledge.

**Clarification:** This is not intended to be a simple `1.4 -> 2.0` replacement - the entire documentation should also be reviewed to ensure it reflects any critical changes between the 1.4 and 2.0 versions

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## AI Summary

### What

Several SQLAlchemy documentation links across the codebase still point at the
**SQLAlchemy 1.4** docs (`https://docs.sqlalchemy.org/en/14/...`), even though
`airflow-core` now **requires SQLAlchemy 2.x**:

- `airflow-core/pyproject.toml` pins `sqlalchemy[asyncio]>=2.0.48`
- `uv.lock` resolves to `sqlalchemy 2.0.51`

The SQLAlchemy 1.4 constraint that originally drove some of these references is
gone, so the user- and developer-facing doc links should point at the `en/20/`
(2.0) docs that match the version Airflow actually runs.

### Acceptance criteria

- [ ] All nine links above use `en/20` and still resolve to a live page + anchor.
- [ ] `set-up-database.rst:201` is left untouched.
- [ ] `breeze build-docs` passes (no broken external-link / spellcheck failures).

### Notes

Doc/comment-only — no behavior change, no newsfragment needed (the
`providers/sqlite` comments are a provider change, so no newsfragment there
either).

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Drafted-by: Claude Code (Opus 4.8); AI Summary section compiled at the maintainer's request

Start by checking airflow-core/pyproject.toml and uv.lock to confirm the SQLAlchemy 2.x versions in use, then locate the nine documentation links that still reference en/14. Review whether the linked pages and anchors remain correct for SQLAlchemy 2.0, leaving set-up-database.rst:201 untouched. Run breeze build-docs and confirm there are no external-link or spellcheck failures.

Written by the indexing model from the issue text.