Fix MR widget with ready to merge buttons/controls at mobile breakpoint

What does this MR do?

EE MR, https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2923

Fix MR widget with ready to merge buttons/controls at mobile breakpoint

State	Before	After
mr_widget_ready_to_merge
mr_widget_merge_when_pipeline_succeeds
mr_widget_merged

Are there points in the code the reviewer needs to double check?

• The MWPS dropdown button will wrap to the next line at 376px. Do we want to prevent wrapping and have it go outside the container? Change the text to MWPS? Not worry about that small of a viewport?, https://i.imgur.com/CiE4btr.png

Why was this MR needed?

MR widget with ready to merge buttons/controls at mobile breakpoint breaking outside of the container.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes #37259 (closed)