Move min ios version to 13.4 for OSS (#36795)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/36795

This change bump the min iOS version for React Native to 13.4, to align with company guidelines.

## Changelog:
[iOS][Changed] - Moved the min iOS version to 13.4

Reviewed By: cortinico

Differential Revision: D44634663

fbshipit-source-id: 035e8fcbb395f7394f8253e3ec485ad9937531c2

packages/rn-tester/NativeCxxModuleExample/NativeCxxModuleExample.podspec

@@ -14,7 +14,7 @@ Pod::Spec.new do |s|
   s.description     = "NativeCxxModuleExample"
   s.homepage        = "https://github.com/facebook/react-native.git"
   s.license         = "MIT"
-  s.platforms       = { :ios => "12.4" }
+  s.platforms       = { :ios => min_ios_version_supported }
   s.compiler_flags  = '-Wno-nullability-completeness'
   s.author          = "Meta Platforms, Inc. and its affiliates"
   s.source          = { :git => "https://github.com/facebook/react-native.git", :tag => "#{s.version}" }