https://gitlab.com/gitlab-org/gitlab/-/merge_requests/38646 had
introduced a bug where `MarkdownContentRewriter` was passed the target's
`#project`. For Epics, this can be `nil`. Instead, we need to pass the
target's `#resource_parent`, which for an Epic, will be a Group.

https://gitlab.com/gitlab-org/gitlab/-/issues/236190