fix left border in session tabs

What does this MR do?

Fixes the missing left border in login session tabs, and removes double border in left tab.

Screenshots

Before:

After:

Does this MR meet the acceptance criteria?

• Changelog entry added
• Tests
  • All builds are passing
• 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 #24941 (closed)