MM-43878: don't err when reply createAt before parent createAt (#20292)

Summary
In a bulk import, if the createAt of a reply is older than the createAt of the parent post, then instead of erroring, we log a warning and set the createAt of the reply to that of the parent post.
I checked the exporting logic, there doesn't appear to be away for the reply createAt to be older than the parent createAt, as a side-effect of the exporting logic.

Ticket Link
https://mattermost.atlassian.net/browse/MM-43878

app/import_functions.go

@@ -1118,6 +1118,10 @@ func (a *App) importReplies(c *request.Context, data []ReplyImportData, post *mo
 		reply.RootId = post.Id
 		reply.Message = *replyData.Message
 		reply.CreateAt = *replyData.CreateAt
+		if reply.CreateAt < post.CreateAt {
+			mlog.Warn("Reply CreateAt is before parent post CreateAt, setting it to parent post CreateAt", mlog.Int64("reply_create_at", reply.CreateAt), mlog.Int64("parent_create_at", post.CreateAt))
+			reply.CreateAt = post.CreateAt
+		}
 		if replyData.Type != nil {
 			reply.Type = *replyData.Type
 		}