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_validators.go

@@ -11,6 +11,7 @@ import (
 	"unicode/utf8"
 
 	"github.com/mattermost/mattermost-server/v6/model"
+	"github.com/mattermost/mattermost-server/v6/shared/mlog"
 )
 
 func validateSchemeImportData(data *SchemeImportData) *model.AppError {
@@ -412,7 +413,7 @@ func validateReplyImportData(data *ReplyImportData, parentCreateAt int64, maxPos
 	} else if *data.CreateAt == 0 {
 		return model.NewAppError("BulkImport", "app.import.validate_reply_import_data.create_at_zero.error", nil, "", http.StatusBadRequest)
 	} else if *data.CreateAt < parentCreateAt {
-		return model.NewAppError("BulkImport", "app.import.validate_reply_import_data.create_at_before_parent.error", nil, "", http.StatusBadRequest)
+		mlog.Warn("Reply CreateAt is before parent post CreateAt", mlog.Int64("reply_create_at", *data.CreateAt), mlog.Int64("parent_create_at", parentCreateAt))
 	}
 
 	return nil