Replaced duplicate z-index with index of 400

What does this MR do?

Adds z-index 400 to header to stop the user dropdown showing underneath the sidebar.

This issue isn't actually present on CE, but this is the backport from EE.

CE port of: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1938

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

closes https://gitlab.com/gitlab-org/gitlab-ce/issues/32609