fix(auth): delegate modular multiFactor to MultiFactorUser constructor

otherwise it delegates to a method that warns of deprecation, because it was
deprecated in namespace API, however for modular API `multiFactor` with specific
parameters is the way to create a MultiFactorUser

Author: mikehardy

packages/auth/lib/modular/index.js

@@ -19,6 +19,7 @@ import { getApp } from '@react-native-firebase/app';
 import { fetchPasswordPolicy } from '../password-policy/passwordPolicyApi';
 import { PasswordPolicyImpl } from '../password-policy/PasswordPolicyImpl';
 import FacebookAuthProvider from '../providers/FacebookAuthProvider';
+import { MultiFactorUser } from '../multiFactor';
 export { FacebookAuthProvider };
 
 /**
@@ -462,7 +463,7 @@ export async function linkWithRedirect(user, provider, resolver) {
  * @returns {MultiFactorUser}
  */
 export function multiFactor(user) {
-  return user._auth.multiFactor(user);
+  return new MultiFactorUser(getAuth(), user);
 }
 
 /**