Remove the limit on Gunicorn dependency (#15611)
Remove the limit on Gunicorn dependency (#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 #15570
sekikn/incubator-airflow,danielvdende/incubator-airflow,danielvdende/incubator-airflow,bolkedebruin/airflow,bolkedebruin/airflow,cfei18/incubator-airflow,nathanielvarona/airflow,cfei18/incubator-airflow,apache/incubator-airflow,danielvdende/incubator-airflow,danielvdende/incubator-airflow,bolkedebruin/airflow,apache/incubator-airflow,apache/airflow,sekikn/incubator-airflow,Acehaidrey/incubator-airflow,lyft/incubator-airflow,lyft/incubator-airflow,apache/incubator-airflow,apache/airflow,Acehaidrey/incubator-airflow,apache/airflow,danielvdende/incubator-airflow,bolkedebruin/airflow,mistercrunch/airflow,apache/airflow,apache/incubator-airflow,cfei18/incubator-airflow,cfei18/incubator-airflow,dhuang/incubator-airflow,Acehaidrey/incubator-airflow,mistercrunch/airflow,mistercrunch/airflow,danielvdende/incubator-airflow,apache/airflow,apache/airflow,dhuang/incubator-airflow,nathanielvarona/airflow,nathanielvarona/airflow,dhuang/incubator-airflow,lyft/incubator-airflow,dhuang/incubator-airflow,nathanielvarona/airflow,nathanielvarona/airflow,lyft/incubator-airflow,cfei18/incubator-airflow,bolkedebruin/airflow,nathanielvarona/airflow,cfei18/incubator-airflow,Acehaidrey/incubator-airflow,Acehaidrey/incubator-airflow,sekikn/incubator-airflow,Acehaidrey/incubator-airflow,sekikn/incubator-airflow,mistercrunch/airflow