Fixes dependencies to pre-release versions of apache-airflow (#11578)

The dependencies in Alphas are currently >= 2.0.0 and should be
>=2.0.0a0 in order to work with Alphas in cases which are not PEP
440-compliant.

According to https://www.python.org/dev/peps/pep-0440/, >= 2.0.0 should
also work with alpha/beta releases (a1/a2) but in some cases it does not
(https://apache-airflow.slack.com/archives/C0146STM600/p1602774750041800)

Changing to ">=2.0.0a0" should help.

Fixes #11577

provider_packages/setup_provider_packages.py

@@ -279,7 +279,7 @@ def do_setup_package_providers(provider_package_id: str,
         airflow_dependency = 'apache-airflow~=1.10' if provider_package_id != 'cncf.kubernetes' \
             else 'apache-airflow>=1.10.12, <2.0.0'
     else:
-        airflow_dependency = 'apache-airflow>=2.0.0'
+        airflow_dependency = 'apache-airflow>=2.0.0a0'
 
     install_requires = [
         airflow_dependency