Make Puma low-level handler return recommended status code

What does this merge request do and why?

Because of https://github.com/puma/puma/pull/3094, Puma v6.4.0 now returns the status code sent from the low-level handler instead of Puma's recommended status code. This caused malformed URLs to return a 500 error when previously it would return a 400 Bad Request error, which skewing error metrics.

Previously we had hard-coded low-level errors to return 500 status errors, but now we should adapt this to return the status code recommended by Puma.

Relates to gitlab-com/gl-infra/production#16417 (closed)

Related merge requests:

• CNG: gitlab-org/build/CNG!1556 (merged)
• Omnibus: omnibus-gitlab!7161 (merged)

gitlab#426135 (closed) has been created to address this duplication.

How to set up and validate locally

With the current gitlab/config/puma.rb:

$ curl -I 'http://gdk.test:3000/-/merge_requests?sort=created_date&state=<th:t=\"%24{dfb}%23foreach'
HTTP/1.1 500 Internal Server Error
Content-Length: 244
Date: Mon, 25 Sep 2023 14:50:50 GMT

Now:

1. Run bundle exec rake gitlab/config/puma.rb.
2. gdk restart rails-web

$ curl -I 'http://gdk.test:3000/-/merge_requests?sort=created_date&state=<th:t=\"%24{dfb}%23foreach'
HTTP/1.1 400 Bad Request
Date: Mon, 25 Sep 2023 14:41:36 GMT

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.