Update MR widget styling

What does this MR do and why?

This MR is meant to fix a styling Issue which was first discovered here: https://gitlab.slack.com/archives/CV2M96LJG/p1661912275654119 It aligns the MR Widget Logo to the center.

UPDATE: Turns out there was already an Issue for this: #372162 (closed)

Screenshots or screen recordings

before	after

How to set up and validate locally

1. Open any MR with the MR widgets in action (example: gitlab-org/frontend/playground/gitlab-mr-widgets-demo!1 (closed))

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.