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_test.go

@@ -887,14 +887,6 @@ func TestImportValidateReplyImportData(t *testing.T) {
 	}
 	err = validateReplyImportData(&data, parentCreateAt, maxPostSize)
 	require.NotNil(t, err, "Should have failed due to 0 create-at value.")
-
-	data = ReplyImportData{
-		User:     ptrStr("username"),
-		Message:  ptrStr("message"),
-		CreateAt: ptrInt64(parentCreateAt - 100),
-	}
-	err = validateReplyImportData(&data, parentCreateAt, maxPostSize)
-	require.NotNil(t, err, "Should have failed due parent with newer create-at value.")
 }
 
 func TestImportValidatePostImportData(t *testing.T) {