Bump version to 0.20.6
Summary: See title.
Test Plan:
make bitcoind --version bitcoin-cli --version bitcoin-qt --version bitcoin-tx -h ninja bitcoind --version bitcoin-cli --version bitcoin-qt --version bitcoin-tx -h
Reviewers: #bitcoin_abc, deadalnix, Fabien
Reviewed By: #bitcoin_abc, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D4299