Mark Webpacker v3-v4 upgrade guide as outdated

The referenced config files no longer exist in React on Rails.
Preserves historical context while directing users to current resources.

Author: ihabadham

docs/additional-details/upgrade-webpacker-v3-to-v4.md

@@ -1,10 +1,12 @@
-# Upgrading rails/webpacker v3.5 to v4
+# Upgrading rails/webpacker v3.5 to v4 (Outdated)
 
-The following steps can be followed to update a Webpacker v3.5 app to v4.
+_Note: This document is outdated. The configuration files referenced below were removed from React on Rails. For current configuration, see the [install generator templates](https://github.com/shakacode/react_on_rails/tree/master/lib/generators/react_on_rails/templates) or consider upgrading to [Shakapacker](https://github.com/shakacode/shakapacker)._
+
+The following steps could be followed to update a Webpacker v3.5 app to v4:
 
 1. Update the gem `webpacker` and the package `@rails/webpacker`
-1. Merge changes from the new default [babel.config.js](https://github.com/shakacode/react_on_rails/tree/master/lib/generators/react_on_rails/templates/base/base/babel.config.js.tt) to your `/babel.config.js`. If you are using React, you need to add `"@babel/preset-react"`, to the list of `presets`.
-1. Copy the file [shakapacker.yml](https://github.com/shakacode/react_on_rails/tree/master/lib/generators/react_on_rails/templates/base/base/config/shakapacker.yml) to `/config/` (Note: newer versions use Shakapacker instead of Webpacker).
-1. Check generator templates in [lib/generators/react_on_rails/templates](https://github.com/shakacode/react_on_rails/tree/master/lib/generators/react_on_rails/templates) for current configuration files.
+1. Merge changes from the new default `.babelrc` to your `/.babelrc`. If you are using React, you need to add `"@babel/preset-react"`, to the list of `presets`.
+1. Copy the file `.browserslistrc` to `/`.
+1. Merge any differences between `config/webpacker.yml` and your `/config/webpacker.yml`.
 
 Here is an [example commit of these changes](https://github.com/shakacode/react_on_rails-tutorial-v11/pull/1/files).