Replace milestone promotion banner with alert

What does this MR do and why?

• Replace the milestone promotion banner with an alert because there is nothing to promote. For information we use alerts.
• Fix bug when burndown chart was visible with no issues in a milestone.
• Fix bug when "no issues" alert was shown with one issue in a milestone.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

When	Before	After
Milestone is ready to be closed
No issues, no dates
No dates
No issues
Issues and dates

How to set up and validate locally

• Go to any empty milestone, for example http://gdk.test:3000/gitlab-org/gitlab-shell/-/milestones
• Create a milestone with no dates
• See a 'Add start and due date to view a burndown chart. Assign some issues to this milestone.' alert
• Add dates
• See 'Assign some issues to this milestone.' alert
• Remove dates
• Assign one issue to this milestone
• See 'Add start and due date to view a burndown chart.' alert
• Add dates
• See burndown/burnup charts and no alerts
• Close assigned issue
• Go back to milestone
• See All issues for this milestone are closed. You may close this milestone now. alert

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.

#424397 (closed)

ee/app/helpers/ee/timeboxes_helper.rb

@@ -14,7 +14,8 @@ def show_burndown_charts_promotion?(milestone)
 
     def show_burndown_placeholder?(milestone)
       milestone.supports_milestone_charts? &&
-        can?(current_user, :admin_milestone, milestone.resource_parent)
+        can?(current_user, :admin_milestone, milestone.resource_parent) &&
+        !milestone.start_date.present? && !milestone.due_date.present?
     end
 
     def milestone_weight_tooltip_text(weight)