Amend references to "OpenSSL license"

A small number of files contain references to the "OpenSSL license"
which has been deprecated and replaced by the "Apache License 2.0".
Amend the occurences.

Fixes #11649

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/11663)

apps/kdf.c

diff --git a/apps/kdf.c b/apps/kdf.c
index 9a69682..8d11807 100644
--- a/apps/kdf.c
+++ b/apps/kdf.c
@@ -1,7 +1,7 @@
 /*
  * Copyright 2019-2020 The OpenSSL Project Authors. All Rights Reserved.
  *
- * Licensed under the OpenSSL license (the "License").  You may not use
+ * 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
  * in the file LICENSE in the source distribution or at
  * https://www.openssl.org/source/license.html