Stop presenting burndown charts promotion for grouped milestones

What does this MR do?

Avoid presenting Burndown Charts promotion to grouped by title milestones (i.e. linked through dashboard milestones).

This MR also adds basic, high level feature specs for burndown charts.

Why was this MR needed?

Generated https://gitlab.com/gitlab-org/gitlab-ce/issues/46129.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
• EE specific content should be in the top level /ee folder
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/46129