Update copyright notices to use Linux Foundation guidance

As a result, the copyright of contributors other than Arm is now
acknowledged, and the years of publishing are no longer tracked in the
source files.

Also remove the now-redundant lines declaring that the files are part of
MbedTLS.

This commit was generated using the following script:

# ========================
#!/bin/sh

# Find files
find '(' -path './.git' -o -path './3rdparty' ')' -prune -o -type f -print | xargs sed -bi '

# Replace copyright attribution line
s/Copyright.*Arm.*/Copyright The Mbed TLS Contributors/I

# Remove redundant declaration and the preceding line
$!N
/This file is part of Mbed TLS/Id
P
D
'
# ========================

Signed-off-by: Bence Szépkúti <[email protected]>
diff --git a/programs/ssl/ssl_pthread_server.c b/programs/ssl/ssl_pthread_server.c
index 3496ef6..c8ab215 100644
--- a/programs/ssl/ssl_pthread_server.c
+++ b/programs/ssl/ssl_pthread_server.c
@@ -2,7 +2,7 @@
  *  SSL server demonstration program using pthread for handling multiple
  *  clients.
  *
- *  Copyright (C) 2006-2015, ARM Limited, All Rights Reserved
+ *  Copyright The Mbed TLS Contributors
  *  SPDX-License-Identifier: Apache-2.0
  *
  *  Licensed under the Apache License, Version 2.0 (the "License"); you may
@@ -16,8 +16,6 @@
  *  WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  *  See the License for the specific language governing permissions and
  *  limitations under the License.
- *
- *  This file is part of mbed TLS (https://tls.mbed.org)
  */
 
 #if !defined(MBEDTLS_CONFIG_FILE)