Remove the limit on Gunicorn dependency

[potiuk]

It seems that the < 20.0 limit for gunicorn was added at some point
in time without actual reason. We are already using gunicorn in
1.10 line of Airflow, so it should not be a problem to bump the
version of gunicorn, especially that the 19. line is somewhat
deprecated already.

This change came after the discussion n #15570

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[potiuk]

Looks like all tests are passing with gunicorn 20.1.0 (the latest one). I tested it locally as well, and everything seems to be fine Airflow works with several gunicorn workers - looks like no problems. I think we can safely merge this one.

[ashb]

I don't feel great about no upper limit here - it means Gunicorn could release a v22 that breaks us.

[uranusjr]

That's when the constraints files is useful 🙂 Python packaging encourages the "compatible unless proven guilty" approach since it's much easier for the user to add extra constraints if gunicorn 22 proves incompatible, but it's much more difficult to remove gunicorn<22 if it causes unnecessarily conflicts.

[potiuk]

Agree with @uranusjr - our constraints approach is designed to prevent precisely this problem and it works with most of our dependencies.

[github-actions]

The PR most likely needs to run full matrix of tests because it modifies parts of the core of Airflow. However, committers might decide to merge it quickly and take the risk. If they don't merge it quickly - please rebase it to the latest master at your convenience, or amend the last commit of the PR, and push it with --force-with-lease.