docs: update SQLAlchemy documentation links from 1.4 to 2.0

[kalyanram07]

This PR updates the outdated SQLAlchemy documentation links across the codebase and documentation files. Since airflow-core now requires SQLAlchemy 2.x, all user- and developer-facing references pointing to the old en/14/ manuals have been updated to point to the current en/20/ documentation tracks.

Closes: #69164

Ensures that developers and users clicking on reference links inside the documentation or code comments are directed to the correct, modern version of the SQLAlchemy documentation matching the active requirements.
Type of Change

• Documentation

[boring-cyborg]

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[shahar1]

Thanks for your contribution!
It's a good start, but by creating the original issue I meant to do a bit more than just replacing 1.4 with 2.0 (I could also do it instead of creating an issue).
We need to actually fact-check that everything in this document is still relevant after the migration.