Add outer border for repository file tree table

What does this MR do?

Adds an outer border for our repository file tree UI. Affecting

• Repository files view (Project overview and Repository overview)
• Find files view
• Project artifact browse view

No border	New border

What are the relevant issue numbers?

Relates to gitlab-design#166 (comment 94170417)

Closes #50264 (closed)

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
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides