fix: remove unused MFA_PATH constant and improve migration guide

Agent-Logs-Url: https://github.com/auth0/Auth0.Android/sessions/3f5dc59e-fa49-4dd5-bb12-215b68803928

Co-authored-by: pmathew92 <17837162+pmathew92@users.noreply.github.com>

Author: Copilot

V4_MIGRATION_GUIDE.md

@@ -120,6 +120,8 @@ The following MFA methods have been removed from `AuthenticationAPIClient`. They
 - `loginWithRecoveryCode(mfaToken, recoveryCode)`
 - `multifactorChallenge(mfaToken, challengeType, authenticatorId)`
 
+Use `AuthenticationAPIClient.mfaClient(mfaToken)` to obtain a `MfaApiClient` instance and handle MFA flows using the new APIs. See the [MFA Flexible Factors Grant](EXAMPLES.md#mfa-flexible-factors-grant) section in `EXAMPLES.md` for usage guidance.
+
 ### DPoP Configuration Moved to Builder
 
 The `useDPoP(context: Context)` method has been moved from the `WebAuthProvider` object to the login

auth0/src/main/java/com/auth0/android/authentication/AuthenticationAPIClient.kt

@@ -982,7 +982,6 @@ public class AuthenticationAPIClient @VisibleForTesting(otherwise = VisibleForTe
         private const val TOKEN_PATH = "token"
         private const val USER_INFO_PATH = "userinfo"
         private const val REVOKE_PATH = "revoke"
-        private const val MFA_PATH = "mfa"
         private const val CHALLENGE_PATH = "challenge"
         private const val PASSKEY_PATH = "passkey"
         private const val REGISTER_PATH = "register"