Refactor Files Header to use `gl-` utility classes
<!--IssueSummary start--> <details> <summary> Everyone can contribute. [Help move this issue forward](https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows/#contributor-links) while earning points, leveling up and collecting rewards. </summary> - [Close this issue](https://contributors.gitlab.com/manage-issue?action=close&projectId=278964&issueIid=276800) </details> <!--IssueSummary end--> The files header component is currently using custom styles. Let's move away from that and apply `gl-` util classes on the components. Make sure to address these too: - [ ] @zcuddy started a [discussion](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/46406#note_441859388): (+1 comment) > **question:** Are we able to utilize utility classes to replace this custom style? I'm worried that it doesn't fit our [GitLab UI spacing scale](https://gitlab.com/gitlab-org/gitlab-ui/-/blob/master/src/scss/variables.scss#L4-20) - [ ] @zcuddy started a [discussion](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/46406#note_441859384): > **question:** Are we able to utilize utility classes to replace this custom style? I'm worried that it doesn't fit our [GitLab UI spacing scale](https://gitlab.com/gitlab-org/gitlab-ui/-/blob/master/src/scss/variables.scss#L4-20) ## Note There are a few areas of our app using `file-headers`, so please make sure to make the changes on **ALL** of them.
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