Migrate milestone.scss to a pagebundle

What does this MR do and why?

Converts EE-only milestone stylesheet into a page bundle.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. You should have a EE license in your GDK.
2. Apply the following patch to see all the elements on the page:

       diff --git a/ee/app/views/shared/milestones/_burndown.html.haml b/ee/app/views/shared/milestones/_burndown.html.haml
    index 6a54d6bcb1a8..73e72a2f1243 100644
    --- a/ee/app/views/shared/milestones/_burndown.html.haml
    +++ b/ee/app/views/shared/milestones/_burndown.html.haml
    @@ -9,18 +9,17 @@
        is_legacy: legacy_milestone?(milestone),
        burndown_events_path: expose_url(burndown_endpoint) } }
    
    -- elsif show_burndown_placeholder?(milestone)
    -  .burndown-hint.content-block.container-fluid
    -    = sprite_icon('close', size: 16, css_class: 'dismiss-icon')
    -    .row
    -      .col-sm-4.col-12.svg-container
    -        = custom_icon('icon_burndown_chart_splash')
    -      .col-sm-8.col-12.inner-content
    -        %h4
    -          Burndown chart
    -        %p
    -          View your milestone's progress as a burndown chart.  Add both a start and a due date to
    -          this milestone and the chart will appear here, always up-to-date.
    +.burndown-hint.content-block.container-fluid
    +  = sprite_icon('close', size: 16, css_class: 'dismiss-icon')
    +  .row
    +    .col-sm-4.col-12.svg-container
    +      = custom_icon('icon_burndown_chart_splash')
    +    .col-sm-8.col-12.inner-content
    +      %h4
    +        Burndown chart
    +      %p
    +        View your milestone's progress as a burndown chart.  Add both a start and a due date to
    +        this milestone and the chart will appear here, always up-to-date.
    
    -        = link_to "Add start and due date", edit_milestone_path(milestone), class: 'btn gl-button'
    +      = link_to "Add start and due date", edit_milestone_path(milestone), class: 'btn gl-button'
    = render 'shared/promotions/promote_burndown_charts', milestone: milestone

3. Open http://localhost:3000/gitlab-org/gitlab-shell/-/milestones/6 in your local environment

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.

Related to #366606 (closed)