Introduce new pipeline details header - part 4

What does this MR do and why?

Fourth and final MR in the effort to build out the new pipeline details header. In this MR we remove the old code, feature flag, flag stubs and update the test coverage.

First MR: !120634 (merged)

Second MR: !121174 (merged)

Third MR: !122216 (merged)

Design can be found here #394768[Unbox-pipeline-detail-page.png]

Screenshots or screen recordings

How to set up and validate locally

1. Visit a project
2. Visit a pipeline
3. View the header section

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.

Related to #394768 (closed)