doc: 2021 is the Year of the Gopher

What a year it has been. If there's one thing we can count on
at a time like this, it's that 2021 is the Year of the Gopher.

Change-Id: I4f6fbbe3d9b4b70df8fadd0d18237f05cd572265
Reviewed-on: https://go-review.googlesource.com/c/go/+/280163
Trust: Dmitri Shuralyov <[email protected]>
Run-TryBot: Dmitri Shuralyov <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Alberto Donizetti <[email protected]>
Reviewed-by: Brad Fitzpatrick <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>

Author: dmitshur

doc/contribute.html

@@ -1023,13 +1023,13 @@ <h3 id="copyright">Copyright headers</h3>
 </p>
 
 <pre>
-// Copyright 2020 The Go Authors. All rights reserved.
+// Copyright 2021 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.
 </pre>
 
 <p>
-(Use the current year if you're reading this in 2021 or beyond.)
+(Use the current year if you're reading this in 2022 or beyond.)
 Files in the repository are copyrighted the year they are added.
 Do not update the copyright year on files that you change.
 </p>