copyright-header: add two expected holder names

Summary:
"The Bitcoin ABC developers" is used in all the buildbot scripts.
"Pieter Wuille, Shammah Chancellor, Neil Booth" is used in cashaddr.py

Depends on D9814

Test Plan:
Before:

$./contrib/devtools/copyright_header.py report .
27 with unexpected copyright holder names

After:

$./contrib/devtools/copyright_header.py report .
0 with unexpected copyright holder names

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D9817