Merge branch 'rs-changelog-markdown' into 'master'

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

[ci skip]

See merge request !6925