Undeprecate private_key option in SFTPHook (#15348)
Undeprecate private_key option in SFTPHook (#15348)

Remove the deprecation warning for the private_key option in SFTPHook. There are valid use cases for storing a private key in the connection object (e.g. storing them in connections backed by an external secrets backend).
Acehaidrey/incubator-airflow,mistercrunch/airflow,bolkedebruin/airflow,danielvdende/incubator-airflow,lyft/incubator-airflow,dhuang/incubator-airflow,apache/airflow,bolkedebruin/airflow,apache/incubator-airflow,Acehaidrey/incubator-airflow,dhuang/incubator-airflow,Acehaidrey/incubator-airflow,apache/incubator-airflow,mistercrunch/airflow,danielvdende/incubator-airflow,nathanielvarona/airflow,apache/airflow,danielvdende/incubator-airflow,dhuang/incubator-airflow,sekikn/incubator-airflow,lyft/incubator-airflow,danielvdende/incubator-airflow,sekikn/incubator-airflow,danielvdende/incubator-airflow,nathanielvarona/airflow,cfei18/incubator-airflow,bolkedebruin/airflow,lyft/incubator-airflow,nathanielvarona/airflow,Acehaidrey/incubator-airflow,cfei18/incubator-airflow,apache/airflow,nathanielvarona/airflow,apache/incubator-airflow,apache/airflow,sekikn/incubator-airflow,bolkedebruin/airflow,nathanielvarona/airflow,mistercrunch/airflow,mistercrunch/airflow,cfei18/incubator-airflow,Acehaidrey/incubator-airflow,bolkedebruin/airflow,apache/airflow,sekikn/incubator-airflow,cfei18/incubator-airflow,Acehaidrey/incubator-airflow,apache/incubator-airflow,lyft/incubator-airflow,apache/airflow,danielvdende/incubator-airflow,cfei18/incubator-airflow,cfei18/incubator-airflow,nathanielvarona/airflow,dhuang/incubator-airflow