fix(ios): Xcode 12 compatibility (#1643)

* fix: add React-Core to podspec for Xcode 12 compatibility

https://github.com/facebook/react-native/issues/29633

* chore: remove React dependency

react-native-webview.podspec

   s.source       = { :git => "https://github.com/react-native-community/react-native-webview.git", :tag => "v#{s.version}" }
   s.source_files  = "apple/**/*.{h,m}"
 
-  s.dependency 'React'
+  s.dependency 'React-Core'
 end