Fix deprecation: render :text is deprecated because it does not actually render…

What does this MR do?

Fixes deprecation: DEPRECATION WARNING: `render :text` is deprecated because it does not actually render a `text/plain` response. Switch to `render plain: 'plain text'` to render as `text/plain`, `render html: '<strong>HTML</strong>'` to render as `text/html`, or `render body: 'raw'` to match the deprecated behavior and render with the default Content-Type, which is `text/html`. (called from render at /builds/gitlab-org/gitlab-ee/app/controllers/application_controller.rb:116)

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Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
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• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.