Replace fa-fire icon with GitLab SVG fire icon

As part of the effort to replace FontAwesome icons with GitLab SVGs we need to use the fire icon instead of the FontAwesome variants.

1 times: fa-fire

The one use is the extension of a special css class called icon-caution (so most probably more things that need replacement there then just one)

When you are done

1. Add screenshots from before + after to make review easier and faster
2. Get UX Review
3. Get Frontend Maintainer review directly

References

• See https://leipert-projects.gitlab.io/is-gitlab-pretty-yet/icons/ for all Font Awesome icon instances
• See http://gitlab-org.gitlab.io/gitlab-svgs/ for all GitLab icons