fix(auth): deprecate MultiFactorUser.enrolledFactor should be plural enrolledFactors (#7652)

* fix!: enrolledFactors rather than enrolledFactor

the typescript definitions specify it as `enrolledFactors` reflecting
that it is an array, but the javascript was actually setting it
as `enrolledFactor` at runtime.

ref:
- typings: https://github.com/invertase/react-native-firebase/blob/main/packages/auth/lib/index.d.ts#L568
- android: https://github.com/invertase/react-native-firebase/blob/main/packages/auth/android/src/main/java/io/invertase/firebase/auth/ReactNativeFirebaseAuthModule.java#L2476
- ios: https://github.com/invertase/react-native-firebase/blob/main/packages/auth/ios/RNFBAuth/RNFBAuthModule.m#L1681
- web sdk: https://github.com/firebase/firebase-js-sdk/blob/master/packages/auth/src/mfa/mfa_user.ts#L34

Author: mnahkies

packages/auth/lib/multiFactor.js

@@ -14,6 +14,8 @@ export class MultiFactorUser {
       user = auth.currentUser;
     }
     this._user = user;
+    this.enrolledFactors = user.multiFactor.enrolledFactors;
+    // @deprecated kept for backwards compatibility, please use enrolledFactors
     this.enrolledFactor = user.multiFactor.enrolledFactors;
   }