Remove the artifacts_management_page feature flag

What does this MR do and why?

For #407475 (closed)

This MR removes the artifacts_management_page feature flag and all references to it. This flag has been enabled on GitLab.com for long enough without causing issues that it should be safe to remove, which will fully enable the artifacts page feature:

• This adds a link to the page in the sidebar: CI/CD => Artifacts (or Build => Artifacts if you're using the new navigation)
• This link should go to a page that lists artifact files by job, shows some details about those artifact files, and allows users with the appropriate permission to delete those artifact files
• The page should not (yet) show selection checkboxes next to artifacts, because that is behind a separate bulk delete feature flag

Screenshots or screen recordings

old sidebar link
new sidebar link
page

How to set up and validate locally

1. check out this branch

git checkout 407475-remove-artifacts-management-page-feature-flag

2. navigate to a project and select CI/CD => Artifacts in the sidebar (or Build => Artifacts if you're using the new navigation)
3. the page should show a list of jobs that have run in the project's pipelines
4. expand a job to see its artifact files and, if you have a sufficient permission level, delete them

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.