Update milestone banner illustration

Changelog: changed
EE: true

ee/app/views/shared/icons/_icon_burndown_charts.svg

-<svg xmlns="http://www.w3.org/2000/svg" width="78" height="82" viewBox="0 0 78 82"><g fill="none" fill-rule="evenodd"><path fill="#F9F9F9" d="M2.12 42c-.08.99-.12 1.99-.12 3 0 20.435 16.565 37 37 37s37-16.565 37-37c0-1.01-.04-2.01-.12-3C74.353 61.032 58.425 76 39 76 19.575 76 3.647 61.032 2.12 42z"/><path fill="#EEE" fill-rule="nonzero" d="M39 78C17.46 78 0 60.54 0 39S17.46 0 39 0s39 17.46 39 39-17.46 39-39 39zm0-4c19.33 0 35-15.67 35-35S58.33 4 39 4 4 19.67 4 39s15.67 35 35 35z"/><path fill="#E1DBF2" fill-rule="nonzero" d="M59.542 39.692l-12.93 15.484a2 2 0 0 1-3.36-.466L31.911 29.34l-9.073 10.532a4.007 4.007 0 0 0-2.9-2.762l10.998-12.767a2 2 0 0 1 3.341.49l11.318 25.318 9.511-11.39A3.989 3.989 0 0 0 58 40c.547 0 1.068-.11 1.542-.308z"/><circle cx="32" cy="26" r="4" fill="#6B4FBB"/><circle cx="45" cy="54" r="4" fill="#6B4FBB"/><path fill="#6B4FBB" fill-rule="nonzero" d="M19 47a6 6 0 1 1 0-12 6 6 0 0 1 0 12zm0-4a2 2 0 1 0 0-4 2 2 0 0 0 0 4zm39-1a6 6 0 1 1 0-12 6 6 0 0 1 0 12zm0-4a2 2 0 1 0 0-4 2 2 0 0 0 0 4z"/></g></svg>
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ee/app/views/shared/promotions/_promote_burndown_charts.html.haml

@@ -10,7 +10,7 @@
         = s_('Promotions|Improve milestones with Burndown Charts.')
 
     - c.with_illustration do
-      = custom_icon('icon_burndown_charts')
+      = image_tag 'illustrations/milestone-sm.svg'
 
     %p
       = s_('Promotions|Burndown Charts are visual representations of the progress of completing a milestone. At a glance, you see the current state for the completion a given milestone. Without them, you would have to organize the data from the milestone and plot it yourself to have the same sense of progress.')