* adds clear information that the provider is for 2.1+
* adds explicit dependency to apache-airflow>=2.1.0 in dependency list
* adds capability of specifying additional dependencies
* different providers now can depend on different Airflow version
* removed pre-commit check and provider info update for
  provider-schema 2.0.0 compatibility
  (not needed any more after >= 2.1.0 is used as Airflow >2.0.1
  allows additional properties in provider_info)
* Update changelog documentation for all providers

Co-authored-by: jarek <jarek@penguini>