PR: https://github.com/go-gitea/gitea/pull/18806
Comments have an id (see Gitea[0], GitLab[1] or GitHub[2]), and the comment migration format must represent it during migrations so that it can be used during mirroring or incremental migrations.
The Index field is added to the Comment table so that it can be used during mirroring to find an existing comment and, for instance, update its content.
[0] https://try.gitea.io/api/swagger#/issue/issueGetComment [1] https://docs.gitlab.com/ee/api/discussions.html#get-single-issue-discussion-item [2] https://docs.github.com/en/rest/reference/issues#get-an-issue-comment