Drop support for oauth2client

[parthea]

googleapis/oauth2client is a is deprecated. It is not listed as a dependency but we still have code and tests that use oauth2client.

As a side benefit, if we drop oauth2client, we would also remove file_cache and the corresponding info log in file_cache here.

[mweinelt]

Oauth2client imports the dead and removed cgi battery, which breaks the test suite on Python 3.13.

______________ test__convert_appengine_app_assertion_credentials _______________

app_identity = <MagicMock name='app_identity' id='140737260342304'>
mock_oauth2client_gae_imports = None

    @mock.patch("google.auth.app_engine.app_identity")
    def test__convert_appengine_app_assertion_credentials(
        app_identity, mock_oauth2client_gae_imports
    ):
    
>       import oauth2client.contrib.appengine  # type: ignore

tests/test__oauth2client.py:119: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

    # Copyright 2014 Google Inc. All rights reserved.
    #
    # Licensed under the Apache License, Version 2.0 (the "License");
    # you may not use this file except in compliance with the License.
    # You may obtain a copy of the License at
    #
    #      http://www.apache.org/licenses/LICENSE-2.0
    #
    # Unless required by applicable law or agreed to in writing, software
    # distributed under the License is distributed on an "AS IS" BASIS,
    # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    # See the License for the specific language governing permissions and
    # limitations under the License.
    
    """Utilities for Google App Engine
    
    Utilities for making it easier to use OAuth 2.0 on Google App Engine.
    """
    
>   import cgi
E   ModuleNotFoundError: No module named 'cgi'

/nix/store/v1xikr9f6z4jm1qp9ncnv2ycarg4gnpz-python3.13-oauth2client-4.1.3/lib/python3.13/site-packages/oauth2client/contrib/appengine.py:20: ModuleNotFoundError