docs: Update oauth keycloak example with new security manager (#50284)

* update oauth example with new security manager

* Update webserver-authentication.rst

---------

Co-authored-by: Yannick Suter <yannick.suter@reprisk.com>

Author: GrumpyCat51

providers/fab/docs/auth-manager/webserver-authentication.rst

@@ -232,15 +232,15 @@ Here is an example of what you might have in your webserver_config.py:
 
 .. code-block:: python
 
-  import os
-  import jwt
-  import requests
   import logging
   from base64 import b64decode
+
+  import jwt
+  import requests
   from cryptography.hazmat.primitives import serialization
-  from flask_appbuilder.security.manager import AUTH_DB, AUTH_OAUTH
-  from airflow import configuration as conf
-  from airflow.www.security import AirflowSecurityManager
+  from flask_appbuilder.security.manager import AUTH_OAUTH
+
+  from airflow.providers.fab.auth_manager.security_manager.override import FabAirflowSecurityManagerOverride
 
   log = logging.getLogger(__name__)
 
@@ -284,11 +284,11 @@ Here is an example of what you might have in your webserver_config.py:
   public_key = serialization.load_der_public_key(key_der)
 
 
-  class CustomSecurityManager(AirflowSecurityManager):
+  class CustomSecurityManager(FabAirflowSecurityManagerOverride):
       def get_oauth_user_info(self, provider, response):
           if provider == "keycloak":
               token = response["access_token"]
-              me = jwt.decode(token, public_key, algorithms=["HS256", "RS256"])
+              me = jwt.decode(token, public_key, algorithms=["HS256", "RS256"], audience="account")
 
               # Extract roles from resource access
               realm_access = me.get("realm_access", {})