Add padding to Promotion on issues board (Fixes #11735)

What does this MR do?

This is an attempted fix for #11735 (closed). It matches the "Improve Issue Boards" promotion item in boards to the "Welcome To Your Issues Board" item that it appears next to, specifically adding a padding border for spacing, making the upgrade your plan button full-width.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created (N/A)
• Code review guidelines (N/A)
• Merge request performance guidelines (N/A)
• Style guides (N/A)
• Database guides (N/A)
• Separation of EE specific content (N/A)

Performance and testing

This should be low risk, its an aesthetic change only.

Before And After

Before	After