Update copyright year

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/14512)

apps/kdf.c

diff --git a/apps/kdf.c b/apps/kdf.c
index 5c33234..c036a1b 100644
--- a/apps/kdf.c
+++ b/apps/kdf.c
@@ -1,5 +1,5 @@
 /*
- * Copyright 2019-2020 The OpenSSL Project Authors. All Rights Reserved.
+ * Copyright 2019-2021 The OpenSSL Project Authors. All Rights Reserved.
  *
  * Licensed under the Apache License 2.0 (the "License").  You may not use
  * this file except in compliance with the License.  You can obtain a copy