Correct the NotificationsEnabled state from 'developer' to 'stable'

https://bugs.webkit.org/show_bug.cgi?id=255545
<rdar://107159906>

Reviewed by Patrick Angle.

The 'NotificationsEnabled' feature flag is not a development flag; it is just
a toggle for A/B testing. It should therefore be in the 'stable' category.

* Source/WTF/Scripts/Preferences/UnifiedWebPreferences.yaml:

Canonical link: https://commits.webkit.org/263038@main

Source/WTF/Scripts/Preferences/UnifiedWebPreferences.yaml

@@ -4529,7 +4529,7 @@ NotificationEventEnabled:
 
 NotificationsEnabled:
   type: bool
-  status: developer
+  status: stable
   category: dom
   humanReadableName: "Notifications"
   humanReadableDescription: "Enable the Notifications API"