content/static: generate constant 2019 copyright year

The generated static.go file was created in 2019 in CL 156321, and
it inherited the generator code from x/tools that uses today's year.
There isn't a need to update the copyright year, so make it a constant.

This change fixes the failing TestStaticIsUpToDate test in 2020,
and prevents it from breaking in every future year.

Fixes golang/go#36360
Updates golang/go#11811

Change-Id: Ie87b1219448f1e288539f95280492b4f95b76798
Reviewed-on: https://go-review.googlesource.com/c/website/+/213121
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>

content/static/gen.go

@@ -11,7 +11,6 @@ import (
 	"fmt"
 	"go/format"
 	"io/ioutil"
-	"time"
 	"unicode"
 )
 
@@ -106,6 +105,6 @@ func appendQuote(out *bytes.Buffer, data []byte) {
 
 const warning = `// Code generated by "makestatic"; DO NOT EDIT.`
 
-var license = fmt.Sprintf(`// Copyright %d The Go Authors. All rights reserved.
+const license = `// Copyright 2019 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.`, time.Now().UTC().Year())
+// license that can be found in the LICENSE file.`