Resolve "Merge Request Tabs Header is missing bottom border"

What does this MR do?

This MR returns the bottom border to the MR Tabs container

What are the relevant issue numbers?

Closes #57160 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the style guides