Relax sqlalchemy version pin and add pre-release test workflow

[mscheltienne]

At my company, we are driving towards compatibility with python 3.14t free-threaded. While investigating the remaining changes necessary, we noticed that:

• Our CIs are happy with sqlalchemy version 2.1.0b2 + sqlmodel 0.0.38 (everything pass, we test mainly DB operations with postgres using psycopg[c] against local dockerized DB and against actual AWS DB).
• The test environment was tricky to build because sqlmodel 0.0.38 (and other versions) have a very conservative "SQLAlchemy >=2.0.14,<2.1.0" constraint, preventing the installation of 2.1.0b2 by default.
• As far as I could tell, sqlmodel seems compatible with SQLAlchemy 2.1.0+

This PR thus proposes 2 changes:

1. Relax the constraint to <2.2.0, offering support for the new SQLAlchemy version soon-to-be-released and by extension to free-threaded python.
2. Add a test job which runs against the latest release of sqlalchemy, including pre-release versions.

[mscheltienne]

Should I run uv lock --upgrade? What is your workflow here? For now I ran it without the --upgrade flag.

[github-actions]

📝 Docs preview

Last commit 175f9db at: https://6588ea3c.sqlmodel.pages.dev

[YuriiMotov]

I think until SQLAlchemy 2.1 released, they can still break things. So, it's not correct to relax the pin now.
Let's wait for the release

[mscheltienne]

We could keep this PR open with "SQLAlchemy>=2.1.0", removing the upper pin and keeping the pre-release job. But that would have to get merged after 2.1.0 is out.