[AIRFLOW-6584] Pin cassandra driver (apache#7194)

3.21.0 release of Cassandra driver
(https://pypi.org/project/cassandra-driver/3.21.0/) broke backwards
compatibility. We need to pin it to 3.20.2

(cherry picked from commit 8ecbcd7)
(cherry picked from commit d0e6421)

setup.py

@@ -199,7 +199,7 @@ def write_version(filename=os.path.join(*["airflow", "git_version"])):
     'azure-mgmt-resource>=2.2.0',
 ]
 cassandra = [
-    'cassandra-driver>=3.13.0',
+    'cassandra-driver>=3.13.0,<3.21.0',
 ]
 celery = [
     'celery~=4.3',