fix(css): rename flex grow shorthand utility

mentioned in commit 5785d58c

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!2417 (merged)

mentioned in merge request status-page!304 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!2418 (merged)

mentioned in merge request gitlab!64091 (merged)

mentioned in merge request gitlab-docs!1958 (merged)

mentioned in merge request gitlab-docs!1959 (closed)