Update release posts handbook
Completes updates related to the automation of release blog posts styling.
Follows https://gitlab.com/gitlab-com/www-gitlab-com/merge_requests/6152 and https://gitlab.com/gitlab-com/www-gitlab-com/merge_requests/6235
Review apps: https://update-handbook-release-posts.about.gitlab.com/handbook/marketing/blog/release-posts/
Edited by username-removed-236961