Remove artifacts from pipeline details entity

Ref: #329798 (closed)

What does this MR do?

This MR removes our artifacts attribute from PipelineDetailsEntity API.

Why are we doing this?

Following up the work in #324003 (closed) where we started to lazy load downloadable artifacts once a user click on it as you can see below 👇

We continue to improve the ~performance of the index pipeline page, by cleaning up unnecessary data exposed by our API:

1. First step was to get rid of has_downloadable_artifacts which is no longer used by the frontend. This was done in !60950 (merged)
2. Next step is to remove those artifacts from being loaded on the pipeline page to improve page loading. !61253 (merged)

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
• [-] Documentation (if required)
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

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• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team