Properly migrate target branch change GitLab comment (#29340)

GitLab generates "system notes" whenever an event happens within the
platform. Unlike Gitea, those events are stored and retrieved as text
comments with no semantic details. The only way to tell whether a
comment was generated in this manner is the `system` flag on the note
type.

This PR adds detection for a new specific kind of event: Changing the
target branch of a PR. When detected, it is downloaded using Gitea's
type for this event, and eventually uploaded into Gitea in the expected
format, i.e. with no text content in the comment.

This PR also updates the template used to render comments to add support
for migrated comments of this type.

ref:
https://gitlab.com/gitlab-org/gitlab/-/blob/11bd6dc826e0bea2832324a1d7356949a9398884/app/services/system_notes/merge_requests_service.rb#L102

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