fix(app, ios): failing to resolve ios sdk from package.json is an error

there should not be a fallback version here, as that behavior is untested, and
the package.json file must be resolved in general

packages/app/RNFBApp.podspec

@@ -1,7 +1,7 @@
 require 'json'
 require './firebase_json'
 package = JSON.parse(File.read(File.join(__dir__, 'package.json')))
-firebase_sdk_version = package['sdkVersions']['ios']['firebase'] || '~> 7.1.0'
+firebase_sdk_version = package['sdkVersions']['ios']['firebase']
 
 Pod::Spec.new do |s|
   s.name                = "RNFBApp"