ios-simulator: change default iphone version (#21148)

Summary:
this is just a suggestion, but as the iPhone 6 is discontinued by Apple
and the new iPhone line will match the X version, it could be good to update the default simulator.

it could also be the iPhone 8, feel free to suggest other changes!

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Pull Request resolved: https://github.com/facebook/react-native/pull/21148

Differential Revision: D9928120

Pulled By: hramos

fbshipit-source-id: 005faa3a8e00b67f98a778c92ecc01b064e14199

local-cli/runIOS/runIOS.js

@@ -410,7 +410,7 @@ module.exports = {
     {
       command: '--simulator [string]',
       description: 'Explicitly set simulator to use',
-      default: 'iPhone 6',
+      default: 'iPhone X',
     },
     {
       command: '--configuration [string]',