[MR Widget Eng] Update core MR Widget extension component to fully support the design guidelines
* [Report regions design guidelines](https://design.gitlab.com/regions/merge-request-reports) defined in the Pajamas Design System.
* **Frontend Docs** for MR Widget Extension (available now): https://docs.gitlab.com/ee/development/new_fe_guide/modules/widget_extensions.html
This epic will track the efforts of updating the component to fully support the design guidelines for Report regions.
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After reviewing individual widget MRs, there were some differences found that need to be fixed in the core component. The issues in the widget below represent those and are tagged with ~"UX debt"
epic