Enable pipeline_editor_mini_graph feature flag by default

What does this MR do and why?

Cleans up the :pipeline_editor_mini_graph and enables it by default. The feature flag adds the pipeline mini graph and linked pipelines to the pipeline status in the pipeline editor page. It is already enabled globally on production.

Documentation is already updated with !73339 (merged).

Related Links

Implementation Issue: #337514 (closed)

Feature Flag Rollout Issue: #342217 (closed)

Screenshots or screen recordings

How to set up and validate locally

1. Disable the pipeline_editor_mini_graph feature flag. (rails c then Feature.disable(:pipeline_editor_mini_graph))
2. Go to the pipeline editor (CI/CD > Editor)
3. Verify that the mini pipeline graph shows up in the pipeline status.

MR acceptance checklist

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