Replace`credentials.valid` check for `credentials.token_state`

New best practice for checking validity of credentials is no longer credentials.valid which is now deprecated (as of google-auth v2.24.0). Instead the google-auth folks recommend using credentials.token_state

All instances of the following:

cloud-sql-python-connector/google/cloud/sql/connector/client.py

Lines 116 to 118 in 4d5e2b3

    
           if not self._credentials.valid: 
        
               request = google.auth.transport.requests.Request() 
        
               self._credentials.refresh(request)

Should be replaced to:

from google.auth.credentials import TokenState

# ...
if not self._credentials.token_state == TokenState.FRESH:
    self._credentials.refresh(google.auth.transport.requests.Request())

	if not self._credentials.valid:
	request = google.auth.transport.requests.Request()
	self._credentials.refresh(request)