Ensure errors.css is interpreted as UTF-8 encoding

Since
https://gitlab.com/gitlab-org/gitlab-foss/-/merge_requests/19096,
we've inlined the `errors.css` by directly including the Sprockets
assets into the HAML. This worked fine as long as the CSS file
remained as a plain ASCII file. However, the switch to the new CSS
pipeline with
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/144479 appears
to have introduced a single UTF-8 character. Since Sprockets loads the
asset via `File.binread`, the contents of this UTF-8 file is forced as
ASCII-8BIT. When this ASCII-8BIT string is combined with other UTF-8
characters, such as Chinese characters, the encoding fails with a 500
error: `incompatible character encodings: ASCII-8BIT and UTF-8
(Encoding::CompatibilityError)`.

To work around this issue, we force the encoding to UTF-8 to ensure
the error page can render.

Relates to
https://gitlab.com/gitlab-com/gl-infra/production/-/issues/17627

Changelog: fixed

app/views/layouts/errors.html.haml

@@ -4,7 +4,7 @@
     %meta{ :content => "width=device-width, initial-scale=1", :name => "viewport" }
     %title= yield(:title)
     %style
-      = Rails.application.assets_manifest.find_sources('errors.css').first.to_s.html_safe
+      = Rails.application.assets_manifest.find_sources('errors.css').first.to_s.force_encoding('UTF-8').html_safe
   %body
     .page-container
       = yield

app/views/layouts/oauth_error.html.haml

@@ -5,7 +5,7 @@
     %title= yield(:title)
     = stylesheet_link_tag 'application_utilities'
     %style
-      = Rails.application.assets_manifest.find_sources('errors.css').first.to_s.html_safe
+      = Rails.application.assets_manifest.find_sources('errors.css').first.to_s.force_encoding('UTF-8').html_safe
     :css
       svg {
         width: 280px;