Modal header titles have an unnecessary top margin

See for example these dialogs:

Vulnerability modal	MR checkout help

This seems to come from our BS 3 -> 4 migration:

• https://gitlab.com/gitlab-org/gitlab-ce/blob/a73f2ec6b58da2708d786dd4326ee5de7bedd402/app/assets/stylesheets/bootstrap_migration.scss#L53-70