docs: update urllib3 RequestMethods docstrings for v2 compatibility

The `urllib3.request` module was removed in urllib3 v2.0.0, and `RequestMethods` was moved to a private module `urllib3._request_methods`.

This commit updates the docstrings in `google/auth/transport/urllib3.py` to remove references to the now-private `urllib3.request.RequestMethods` class. Instead, it references the public `urllib3.PoolManager` class or the generic request interface, ensuring the documentation remains accurate for both urllib3 v1 and v2 users.

This addresses the documentation aspect of PR #1290.

Author: google-labs-jules[bot]

google/auth/transport/urllib3.py

@@ -106,9 +106,8 @@ class Request(transport.Request):
         credentials.refresh(request)
 
     Args:
-        http (urllib3.request.RequestMethods): An instance of any urllib3
-            class that implements :class:`~urllib3.request.RequestMethods`,
-            usually :class:`urllib3.PoolManager`.
+        http (urllib3.PoolManager): An instance of a urllib3 class that implements
+            the request interface (e.g. :class:`urllib3.PoolManager`).
 
     .. automethod:: __call__
     """
@@ -209,7 +208,7 @@ class AuthorizedHttp(RequestMethods):  # type: ignore
         response = authed_http.request(
             'GET', 'https://www.googleapis.com/storage/v1/b')
 
-    This class implements :class:`urllib3.request.RequestMethods` and can be
+    This class implements the urllib3 request interface and can be
     used just like any other :class:`urllib3.PoolManager`.
 
     The underlying :meth:`urlopen` implementation handles adding the