Update milestone plan script to work with the new release schedule
Use the new SSoT for the release dates https://gitlab.com/gitlab-com/www-gitlab-com/-/raw/master/data/releases.yml.
Thankfully, the functions were already in place in gitlab-docs, so I had to change very little
- https://gitlab.com/gitlab-org/gitlab-docs/-/blob/2818430b1770a6e403250a72bebde1c9f8745f9b/lib/tasks/task_helpers.rb#L138-145
- https://gitlab.com/gitlab-org/gitlab-docs/-/blob/2818430b1770a6e403250a72bebde1c9f8745f9b/lib/helpers/generic.rb#L203-216
Closes #947 (closed)