Cap gunicorn below 25.1.0 to avoid API server startup deadlocks

[kaxil]

Summary

• Temporarily cap the gunicorn extra in airflow-core to gunicorn>=23.0.0,<25.1.0.
• Add an inline comment explaining this is a temporary guard for the API server preload+fork startup deadlock seen with Gunicorn 25.1.0.
• Link directly to upstream tracking and proposed fix for follow-up cleanup.

Context

This is a temporary safety restriction for the Airflow Gunicorn API server path while upstream finalizes and ships a fix:

• Possible deadlock in 25.1.0 when preload_app=True during worker boot benoitc/gunicorn#3529
• fix(ctl): prevent fork deadlock and file watcher triggers benoitc/gunicorn#3520