Avatars > Update fallback avatars to use correct typography

Typography for avatars was not utilizing text styles, which meant some were use line heights that dont match our scale and one was using a type size that we don't have. Rather than creating specific exceptions for avatars, we should align the text to a value that already exists within our type scale.

Spec previews have been updated here: https://gitlab-org.gitlab.io/gitlab-design/hosted/design-gitlab-specs/avatar-spec-previews/