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 <bence.szepkuti@arm.com>

tests/suites/main_test.function

@@ -15,8 +15,6 @@
  *      Test suite file     : $test_case_file
  *      Test suite data     : $test_case_data_file
  *
- *
- *  This file is part of Mbed TLS (https://tls.mbed.org)
  */
 
 #if defined(__unix__) || (defined(__APPLE__) && defined(__MACH__))