Fix the following style-lint errors and warnings for `app/assets/stylesheets/pages/milestone.scss`

Problem

Fix the following style-lint errors and warnings for app/assets/stylesheets/pages/milestone.scss


  34:7  ⚠  Expected ".milestones .milestone .milestone-progress a" to have no more than 3 compound selectors                                                                                             selector-max-compound-selectors
  61:5  ⚠  Expected ".milestone-content .issuable-row span a" to have no more than 3 compound selectors                                                                                                  selector-max-compound-selectors
  99:7  ⚠  Expected ".milestone-sidebar .milestone-progress .sidebar-collapsed-icon .progress" to have no more than 3 compound selectors                                                                 selector-max-compound-selectors
 148:1  ⚠  Selector should be written in lowercase with hyphens                                                                                                                                          selector-class-pattern
 238:3  ⚠  ".popover-body,   .popover-content" and ".popover (app/assets/stylesheets/pages/milestone.scss -234:3)" have the same        stylelint-gitlab/duplicate-selectors
           properties.                                                           selector-max-compound-selectors

What to do

  1. If possible, replace the CSS class with a utility one, check common.scss and https://getbootstrap.com/docs/4.3/utilities/ for existing ones
  2. Double check if all the warning have been fixed by running yarn run stylelint-file app/assets/stylesheets/pages/milestone.scss
  3. In the MR, add screenshots of the affected areas before and after the changes, check the first MR as a guideline
  4. If there are a lot of changes, we recommend to break them into smaller MRs
  5. Double check the usage of each changed class both in CE and EE
  6. EE changes should either be backported into CE or moved into ee/ folder. You can read more about how to handle EE code here
  7. Label the MR with CSS cleanup