Fix double scrollbar issue in milestone selector

This commit fixes a CSS issue that caused the milestones selector to
show two scrollbars.

app/assets/stylesheets/components/milestone_combobox.scss

-.selected-item {
+.milestone-combobox {
+  .selected-item {
     /* stylelint-disable-next-line function-url-quotes */
     background: url(asset_path('checkmark.png')) no-repeat 0 2px;
-}
+  }
 
-.dropdown-item-space {
+  .dropdown-item-space {
     padding: 8px 12px;
+  }
+
+  .dropdown-menu.show {
+    overflow: hidden;
+  }
 }

changelogs/unreleased/nfriend-fix-milestone-combobox-double-scrollbar.yml

+---
+title: Fix double scrollbar issue in milestone selector dropdown
+merge_request: 54734
+author:
+type: fixed