fix: use 2026 copyright year in new test file

Signed-off-by: Pedro Albuquerque <pedroalb@gmail.com>
Signed-off-by: Pedro Albuquerque <pedro.albuquerque@slack-corp.com>

Author: pedroalb

go/vt/vttablet/tabletmanager/vdiff/source_pk_cols_ordering_test.go

@@ -1,5 +1,5 @@
 /*
-Copyright 2025 The Vitess Authors.
+Copyright 2026 The Vitess Authors.
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.