Fixes issue boards list colored top border visual glitch

What does this MR do?

This fixes some visual glitches on board lists borders.

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

We are not sure to have checked all variants of the headers. Please also verify the results with very bright colors (#ff0-bright).

Why was this MR needed?

For purely visual reasons.

Screenshots (if relevant)

This MR changes this… into this…

Does this MR meet the acceptance criteria?

• Changelog entry added
• 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?

• gitlab-org/gitlab-ce#25260 «Board List with Border has bad visual artifacts»