Burndown chart on group milestone page in Starter

• The burndown chart on the group milestone page is currently in Premium.
• This issue is to bring the feature to Starter.
• This simplifies the feature from a tier perspective and also is inline with organizations who are starting to adopt DevOps. We will then say, "the burndown chart feature in GitLab is a Starter feature". And so the burndown chart (whether the project milestone one or the group milestone one, will be a Starter feature).
• As we iterate on GitLab, we plan to add more analytics features in addition to burndown charts to higher tiers, in keeping with organizations that are scaling or optimizing their DevOps adoption.