Fix for iOS Firebase API changes

[mravn-google]

Fixes for iOS Firebase API changes: https://firebase.google.com/support/release-notes/ios

TBR

[mravn-google]

Committing to fix build, TBR

[szakarias]

LGTM

[bizz84]

@mravn-google @szakarias Firestore example app doesn't compile on iOS for me due to this change.

Log:

          [document setData:call.arguments[@"data"] merge:YES completion:defaultCompletionBlock];
           ~~~~~~~~ ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    /Users/andrea/dev/WORK/MuseVisions/flutter/plugins/packages/cloud_firestore/ios/Classes/CloudFirestorePlugin.m:438:14: error: no visible @interface for 'FIRWriteBatch' declares the selector 'setData:forDocument:merge:'
          [batch setData:call.arguments[@"data"] forDocument:document merge:YES];
           ~~~~~ ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

After flutter packages get my Podfile.lock file appears to be using an old FirebaseFirestore version:

  - Firebase/Firestore (4.13.0):
    - Firebase/Core
    - FirebaseFirestore (= 0.11.0)

Are some changes needed to ensure the latest iOS pod version is used?

[szakarias]

LGTM

[bizz84]

This PR goes in tandem with this change on the iOS sdk: https://github.com/firebase/firebase-ios-sdk/pull/1098/files

However, this is available on the iOS sdk 5.0 or above. By default any new Flutter projects using cloud_firestore will incorrectly point to 4.13, so it won't compile for iOS.

I'm not sure how the Firebase plugins choose which underlying sdk versions to use, but it seems like a version should be bumped somewhere for this to work.