Add missing bottom border to blame age legend

What does this MR do?

Adds missing bottom border to blame age legend.

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

None

Why was this MR needed?

Border went missing while blame age legend was under development.

No changelog since it fixes minor display defect from MR from earlier in same day.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• N/A Changelog entry added, if necessary
• N/A Documentation created/updated
• N/A API support added
• Tests
  • N/A Added for this feature/bug
  • All builds are passing
• N/A 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 #33739 (closed)