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/serializers/rest/instance_serializer.rb

@@ -32,7 +32,7 @@ class REST::InstanceSerializer < ActiveModel::Serializer
   end
 
   def thumbnail
-    instance_presenter.thumbnail ? full_asset_url(instance_presenter.thumbnail.file.url) : full_pack_url('preview.jpg')
+    instance_presenter.thumbnail ? full_asset_url(instance_presenter.thumbnail.file.url) : full_pack_url('media/images/preview.jpg')
   end
 
   def stats

app/serializers/rss/account_serializer.rb

@@ -11,7 +11,7 @@ class RSS::AccountSerializer
     builder.title("#{display_name(account)} (@#{account.local_username_and_domain})")
            .description(account_description(account))
            .link(TagManager.instance.url_for(account))
-           .logo(full_asset_url(asset_pack_path('logo.svg')))
+           .logo(full_pack_url('media/images/logo.svg'))
            .accent_color('2b90d9')
 
     builder.image(full_asset_url(account.avatar.url(:original))) if account.avatar?

app/serializers/rss/tag_serializer.rb

@@ -12,7 +12,7 @@ class RSS::TagSerializer
     builder.title("##{tag.name}")
            .description(strip_tags(I18n.t('about.about_hashtag_html', hashtag: tag.name)))
            .link(tag_url(tag))
-           .logo(full_asset_url(asset_pack_path('logo.svg')))
+           .logo(full_pack_url('media/images/logo.svg'))
            .accent_color('2b90d9')
 
     statuses.each do |status|