Create sprockets manifest for example app

https://github.com/rails/sprockets/blob/070fc01947c111d35bb4c836e9bb71962a8e0595/UPGRADING.md#manifestjs

Otherwise
```
** Invoke generate:stuff (first_time)
** Execute generate:stuff
bin/rake app:template LOCATION='../../example_app_generator/generate_stuff.rb'
rake aborted!
Sprockets::Railtie::ManifestNeededError: Expected to find a manifest file in `app/assets/config/manifest.js`
But did not, please create this file and use it to link any assets that need
to be rendered by your app:

Example:
  //= link_tree ../images
  //= link_directory ../javascripts .js
  //= link_directory ../stylesheets .css
and restart your server
```

This, however, fails on Rails < 7:

```
      # --- Caused by: ---
     # Sprockets::Rails::Helper::AssetNotPrecompiled:
     #   application.css
```

Author: pirj

example_app_generator/generate_app.rb

@@ -78,4 +78,8 @@
             'REPLACE_BUNDLE_PATH',
             bundle_install_path
   chmod 'ci_retry_bundle_install.sh', 0755
+
+  if Rails::VERSION::STRING > '7'
+    create_file 'app/assets/config/manifest.js'
+  end
 end