Migrate ee/app/assets/javascripts/environments_dashboard/components/dashboard/environment_header.vue to use design tokens in icon

Currently this component uses this styling from app/assets/stylesheets/page_bundles/operations.scss to color the icon, it should use an icon variant

.dashboard-card {
{...}
  &-icon {
    color: var(--gray-300, $gray-300);
  }