Update checkboxes layout in the merge request merge widget

This is a follow-up from the MR Merge Widget new Framework: Update merged, c... (#368039 - closed)

Changes to be made

1. On 768px + breakpoints, the checkboxes are inline and all items have 16px spacing between them.
2. On <768px breakpoints, the checkboxes are stacked and all items have 8px spacing between them.

🖼 Design specs in Figma