Alert > Updates to match design assets
Background
In #1648 (closed), #1648 (closed), and gitlab-org/gitlab-ui!3706 (merged) the alert heading scale was updated in sync with other components headings (like modal and banner). There was another update in gitlab-org/gitlab-ui!4496 (merged) that reduced the heading size, however, this change will update it back to the earlier design to keep it in sync with the rest of the system. This applies to both the Vue and Rails components.
Additionally, the icon color in the Rails component needs updated to match alert variant colors.