Add system notes metadata to GraphQL NoteType

What does this MR do and why?

Adds description version changes diff and some temporary REST API backward compatibility data to the corresponding system notes data.

Sample GraphQL query

{
  workItem(id: "gid://gitlab/WorkItem/81435") {
    widgets {
      ... on WorkItemWidgetNotes {
        type
        discussions {
          nodes {
            id
            notes {
              nodes {
                id
                body
                bodyHtml
                systemNoteIconName
                createdAt
                systemNoteMetadata {
                  id
                  action
                  descriptionVersion {
                    id
                    description
                    diff
                    diffPath
                    deletePath
                    canDelete
                    deleted
                  }
                }
                author {
                  id
                  avatarUrl
                  name
                  username
                  webUrl
                }
              }
            }
          }
        }
      }
    }
  }
}

#385535 (comment 1207525117)

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