Resolve "skeleton placeholder on diff has white background"

What does this MR do?

The skeleton placeholder lines used to have 6 elements, but only drew 3 lines. Now .skeleton-line-X are actually lines

• remove animation-container background color, so we can put lines in other colored text areas
• Move animation from container to lines
• change colors to match https://gitlab-org.gitlab.io/design.gitlab.com/components/skeleton-loader

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

~bug

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

Closes #44294 (closed)