Bump version to 0.20.0

Summary: See title (and release-process.md).

Test Plan:

make check
ninja

# for both builds:
bitcoind --version
bitcoin-cli --version
bitcoin-tx -h
bitcoin-qt --version

Reviewers: #bitcoin_abc, deadalnix, Mengerian

Reviewed By: #bitcoin_abc, Mengerian

Subscribers: Mengerian

Differential Revision: https://reviews.bitcoinabc.org/D3838