Resolve ""Add Members" drop-down doesn't separate Name/Username"

What does this MR do?

It fixes + aligns the styling of multi select user items. Spacing + Bold + @ sign changed. I thought about aligning it with the styling of single select but the avatar has s24 on purpose so it looks like a deliberate decision to have the one line format (which makes sense cause you see more results at once).

Screenshots (if relevant)

Before :

After :

Does this MR meet the acceptance criteria?

• Documentation created/updated
• 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 #33981 (moved)