MR Merge Widget new Framework: Update merged, closed and ready to merge states

As a follow-up to changes made to the MR merge widget in Restructure MR merge widget (&5589 - closed), we need to adapt it to the new framework design guidelines.

🖼 Design specs in Figma

Summary of the changes

1. The merged and closed states have a unique style:
   1. Their background color is blue-50 or red-50 respectively.
   2. The background color of the details section is white.
2. The ready to merge state contents are responsive:
   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.

(Please beware of the dark mode of GitLab. Please take it for a spin.)

See the epic for more context