Update default Podfile to not depend on a path (#28572)

Summary:
Recently, a default Podfile has been modified to not contain all the React Native pods, but use a helper method `use_react_native!`.

While this is great, it assumes a hardcoded path of `../node_modules/react-native` to be always the correct location of the React Native.

https://github.com/facebook/react-native/blob/d4d8887b5018782eeb3f26efa85125e6bbff73e4/scripts/autolink-ios.rb#L7-L9

Unfortunately, due to the way Ruby works, this completely hides the path away from the users.

Before, they could have seen the wrong path explicitly in a Podfile and knew to update it to resolve path-related issues.

With the current version in `master`, I can see a lot of issues where developers wonder how to resolve the path issues and how to pass the path itself.

https://github.com/facebook/react-native/blob/4118d798265341061105f3a53550db83c66a71cb/template/ios/Podfile#L5-L10

This PR uses React Native CLI configuration (that is already used to link 3rd party dependencies) to explicitly define the correct path to the React Native.

As a result, we don't have to change the paths here whether we're running monorepo or not.

## Changelog

[IOS] [INTERNAL] - Always provide an explicit path to React Native
Pull Request resolved: #28572

Differential Revision: D20945194

Pulled By: TheSavior

fbshipit-source-id: 010f9754f2ed78ef62fd52f4d201f296f5af6d27

template/ios/Podfile

@@ -4,16 +4,15 @@ require_relative '../node_modules/@react-native-community/cli-platform-ios/nativ
 platform :ios, '10.0'
 
 target 'HelloWorld' do
-  # Pods for HelloWorld
-  use_react_native!
+  config = use_native_modules!
+
+  use_react_native!(:path => config["reactNativePath"])
 
   target 'HelloWorldTests' do
     inherit! :complete
     # Pods for testing
   end
 
-  use_native_modules!
-
   # Enables Flipper.
   #
   # Note that if you have use_frameworks! enabled, Flipper will not work and