Deprecate default_flags in Podfile (#39389)

Summary:
Pull Request resolved: facebook/react-native#39389

The Podfile of the Template is using a function called `get_default_flags` to get the default flags.
Its behavior is duplicated in both the default value of the `use_react_native!` function and in the body of the same function, making that helper actually redundant.

In this change, we are deprecating it so we can remove it in 0.74 with no breakages.

## Changelog:
[iOS][Deprecated] - Deprecate `get_default_flags` in Ruby scripts

Reviewed By: dmytrorykun

Differential Revision: D49147290

fbshipit-source-id: 41a9f9aa4ba5d1a31d86953fe78778b45d28d9b2

Original: facebook/react-native@f60b9f6

template/template/ios/Podfile

@@ -28,14 +28,8 @@ end
 target 'HelloWorld' do
   config = use_native_modules!
 
-  # Flags change depending on the env values.
-  flags = get_default_flags()
-
   use_react_native!(
     :path => config[:reactNativePath],
-    # Hermes is now enabled by default. Disable by setting this flag to false.
-    :hermes_enabled => flags[:hermes_enabled],
-    :fabric_enabled => flags[:fabric_enabled],
     # Enables Flipper.
     #
     # Note that if you have use_frameworks! enabled, Flipper will not work and