Deprecate default_flags in Podfile (facebook#39389)

Summary:
Pull Request resolved: facebook#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: 2f9f024084b3b922582d70e7baccb23a389b0caa

packages/react-native/scripts/cocoapods/utils.rb

@@ -16,6 +16,9 @@ def self.warn_if_not_on_arm64
         end
     end
 
+    # deprecated. These checks are duplicated in the react_native_pods function
+    # and we don't really need them. Removing this function will make it easy to
+    # move forward.
     def self.get_default_flags
         flags = {
             :fabric_enabled => false,

packages/react-native/scripts/react_native_pods.rb

@@ -217,7 +217,10 @@ def install_modules_dependencies(spec, new_arch_enabled: ENV['RCT_NEW_ARCH_ENABL
 end
 
 # It returns the default flags.
+# deprecated.
 def get_default_flags()
+  warn 'get_default_flags is deprecated. Please remove the keys from the `use_react_native!` function'
+  warn 'if you are using the default already and pass the value you need in case you don\'t want the default'
   return ReactNativePodsUtils.get_default_flags()
 end
 

packages/react-native/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