Remove the limit on Gunicorn dependency (apache#15611)

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 apache#15570

setup.cfg

@@ -99,7 +99,7 @@ install_requires =
     flask-login>=0.3, <0.5
     flask-wtf>=0.14.3, <0.15
     graphviz>=0.12
-    gunicorn>=19.5.0, <20.0
+    gunicorn>=19.5.0
     importlib_metadata~=1.7;python_version<"3.9" # We could work with 3.1, but argparse needs <2
     importlib_resources~=1.4
     iso8601>=0.1.12