Convert CHANGELOG to Markdown

All this does is convert the version sections into headers. The list items shouldn't really be indented by two spaces, but it makes no difference to the rendering and this way we retain authorship history for the actual changes.

Related to https://gitlab.com/gitlab-org/release-tools/merge_requests/29

Status changed to merged

Mentioned in commit 11da7594

cc @rymai

Mentioned in merge request gitlab-com/www-gitlab-com!3506 (merged)

Mentioned in commit gitlab-com/www-gitlab-com@37396c58

Mentioned in issue #23551 (closed)