<!--

Welcome to Apache Airflow!  For a smooth issue process, try to answer the following questions.
Don't worry if they're not all applicable; just try to include what you can :-)

If you need to include code snippets or logs, please put them in fenced code
blocks.  If they're super-long, please use the details tag like
<details><summary>super-long log</summary> lots of stuff </details>

Please delete these comment blocks before submitting the issue.

-->

**Description**

I want to specify the cipher to use when creating a SftpHook. Currently this is not possible because there is not a way to propagate the value to CnOpts. 

https://github.com/apache/airflow/blob/master/airflow/providers/sftp/hooks/sftp.py
https://pysftp.readthedocs.io/en/release_0.2.9/pysftp.html?#pysftp.CnOpts
https://pysftp.readthedocs.io/en/release_0.2.9/cookbook.html

**Use case / motivation**

<!-- What do you want to happen?

Rather than telling us how you might implement this solution, try to take a
step back and describe what you are trying to achieve.

-->

I want to be able to access SFTP servers that have disabled access from connection with weak ciphers.

airflow/providers/sftp/hooks/sftp.py
warnings.warn(
                        'Extra option `private_key` is deprecated.'
                        'Please use `key_file` instead.'
                        'This option will be removed in Airflow 2.1',
                        DeprecationWarning,
                        stacklevel=2,
                    )
                    self.key_file = extra_options.get('private_key')
    def get_conn(self) -> pysftp.Connection:
        """Returns an SFTP connection object"""
        if self.conn is None:
            cnopts = pysftp.CnOpts()
            if self.no_host_key_check:
                cnopts.hostkeys = None
            cnopts.compression = self.compress
            conn_params = {
                'host': self.remote_host,
                'port': self.port,
                'username': self.username,
                'cnopts': cnopts,
            }
            if self.password and self.password.strip():
                conn_params['password'] = self.password
            if self.key_file:
                conn_params['private_key'] = self.key_file
            if self.private_key_pass:
                conn_params['private_key_pass'] = self.private_key_pass
            self.conn = pysftp.Connection(**conn_params)
        return self.conn
    def close_conn(self) -> None: