Removed horizontal overflow from collapsed issuable sidebar

What does this MR do?

Uses flexbox to remove any horizontal overflow from the collapsed issuable sidebar

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

Why was this MR needed?

There was a horizontal scrollbar at the bottom of all collapsed issuable sidebars as seen in the screenshot in #21895 (closed).

Screenshots (if relevant)

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 you do - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #21895 (closed).