Sync railties release notes [ci skip]

guides/source/4_2_release_notes.md

@@ -69,6 +69,46 @@ Please refer to the [Changelog][railties] for detailed changes.
 
 ### Notable changes
 
+*   Added a `required` option to the model generator for associations.
+    ([Pull Request](https://github.com/rails/rails/pull/16062))
+
+*   Introduced an `after_bundle` callbacks for use in Rails templates.
+    ([Pull Request](https://github.com/rails/rails/pull/16359))
+
+*   Introduced the `x` namespace for defining custom configuration options:
+
+        # config/environments/production.rb
+        config.x.payment_processing.schedule = :daily
+        config.x.payment_processing.retries  = 3
+        config.x.super_debugger = true
+
+    These options are then available through the configuration object:
+
+        Rails.configuration.x.payment_processing.schedule # => :daily
+        Rails.configuration.x.payment_processing.retries  # => 3
+        Rails.configuration.x.super_debugger              # => true
+        Rails.configuration.x.super_debugger.not_set      # => nil
+
+    ([Commit](https://github.com/rails/rails/commit/611849772dd66c2e4d005dcfe153f7ce79a8a7db))
+
+*   Introduced `Rails::Application.config_for` to load a configuration for the
+    current environment.
+
+        # config/exception_notification.yml:
+        production:
+          url: http://127.0.0.1:8080
+          namespace: my_app_production
+        development:
+          url: http://localhost:3001
+          namespace: my_app_development
+
+        # config/production.rb
+        MyApp::Application.configure do
+          config.middleware.use ExceptionNotifier, config_for(:exception_notification)
+        end
+
+    ([Pull Request](https://github.com/rails/rails/pull/16129))
+
 *   Introduced `--skip-gems` option in the app generator to skip gems such as
     `turbolinks` and `coffee-rails` that does not have their own specific flags.
     ([Commit](https://github.com/rails/rails/commit/10565895805887d4faf004a6f71219da177f78b7))