Frontend: Create a single consistent style for the pipeline mini graph

Description

Currently each our mini pipeline graph has a slightly different appearance depending on where it's rendered:

commit	MR widget	pipeline list

The discrepancy is only a couple of px:

MR widget	Pipeline List

Technical Details

• The commit pipeline should be updated by: !55363 (merged)
• The class mr-widget-pipeline-stages should be removed as part of this fix.
• In an old commit we have a leftover comment mentioning this should be addressed a some point:
  • gitlab-foss@bc5a9206

Original Suggestion

The following discussion from !55199 (merged) should be addressed:

• @mrincon started a discussion:

  Suggestion (for follow-up): The mr-widget-pipeline-stages class is responsible for making the mini pipeline just a couple of pixels smaller per stage, and this style could be remove in favor of having a single consistent look for it.

  I'll discuss this with our UX team.