Bump webpacker from 3.5.5 to 4.0.2 (#10277)

Bumps [webpacker](https://github.com/rails/webpacker) from 3.5.5 to 4.0.2.
- [Release notes](https://github.com/rails/webpacker/releases)
- [Changelog](https://github.com/rails/webpacker/blob/master/CHANGELOG.md)
- [Commits](https://github.com/rails/webpacker/compare/v3.5.5...v4.0.2)

Signed-off-by: dependabot[bot] <support@dependabot.com>
Co-authored-by: Yamagishi Kazutoshi <ykzts@desire.sh>

app/views/home/index.html.haml

@@ -11,7 +11,7 @@
 
 .app-holder#mastodon{ data: { props: Oj.dump(default_props) } }
   %noscript
-    = image_tag asset_pack_path('logo.svg'), alt: 'Mastodon'
+    = image_pack_tag 'logo.svg', alt: 'Mastodon'
 
     %div
       = t('errors.noscript_html', apps_path: 'https://joinmastodon.org/apps')