devtools: ensure that copyright year is up to date in create_migration

Change-Id: I6a3fe4d964269c1132dd5c8f5a26fa6d8abb8b80
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/721061
Reviewed-by: Robert Findley <[email protected]>
kokoro-CI: kokoro <[email protected]>
Auto-Submit: Ethan Lee <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: ethanalee-work

devtools/create_migration.sh

@@ -7,7 +7,7 @@
 # Script for creating a new migration file.
 
 migrate create -ext sql -dir migrations -seq $1
-HEADER="-- Copyright 2021 The Go Authors. All rights reserved.
+HEADER="-- Copyright $(date +%Y) The Go Authors. All rights reserved.
 -- Use of this source code is governed by a BSD-style
 -- license that can be found in the LICENSE file.