Use default column background color for board new list

What does this MR do and why?

This removes some custom styling to default new lists on the Boards pages (Issue & Epic) to utilize the default background color (gray-50).

MR acceptance checklist

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Screenshots or screen recordings

State	Before	After
New list (updated bg)
Board column (unchanged)
Swimlanes (unchanged)

How to set up and validate locally

1. Go to a Boards page (Issue Boards or Epic Boards)
2. Click the "Create list" button
3. Verify the new list background is using gray-50 (like other lists)