chore: don't set the new architecture by default for macOS yet (microsoft#2423)

## Summary:

This reverts commit b9f3186.

The new architecture isn't ready on macOS yet. Specifically, the Fabric
renderer isn't ready. This is causing issue for new projects off the
macOS template. Let's disable it until we're ready.
## Test Plan:

CI should pass

Author: Saadnajmi

packages/react-native/scripts/cocoapods/__tests__/new_architecture-test.rb

@@ -407,10 +407,10 @@ def test_newArchEnabled_whenRCTNewArchEnabledIsSetTo0_returnFalse
         assert_false(is_enabled)
     end
 
-    def test_newArchEnabled_whenRCTNewArchEnabledIsNotSet_returnTrue
+    def test_newArchEnabled_whenRCTNewArchEnabledIsNotSet_returnFalse  # [macOS] Disable Fabric by default till macOS supports it]
         ENV["RCT_NEW_ARCH_ENABLED"] = nil
         is_enabled = NewArchitectureHelper.new_arch_enabled
-        assert_true(is_enabled)
+        assert_false(is_enabled)
     end
 
 

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

@@ -189,6 +189,6 @@ def self.compute_new_arch_enabled(new_arch_enabled, react_native_version)
     end
 
     def self.new_arch_enabled
-        return ENV["RCT_NEW_ARCH_ENABLED"] == nil || ENV["RCT_NEW_ARCH_ENABLED"] == "1"
+        return ENV["RCT_NEW_ARCH_ENABLED"] == "1" # [macOS] Disable Fabric by default till macOS supports it
     end
 end