feat!(google-auth): require 'audience' in verify_firebase_token() #18870

Currently, google.oauth2.id_token.verify_firebase_token() (and its async counterpart in google.oauth2._id_token_async) defaults audience=None, leaving the token's aud claim unverified and emitting a UserWarning (added in #18777).

In the next major release of google-auth (3.0.0), audience (the target Firebase project ID) should be required so that verify_firebase_token() always verifies the aud claim per the Firebase ID token verification specification.