Update copyright year to 2021

Summary: Update year in top level COPYING file and build config

Test Plan:
Checked that it shows 2021 with these commands:

ninja
./src/bitcoind --version
./src/qt/bitcoin-qt --version

Reviewers: Fabien, #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D8886