Bump version to 0.20.4

Summary:
See title.
Also started defaulting to a non-empty release notes.

Test Plan:

make check
bitcoind --version
bitcoin-cli --version
bitcoin-tx -h
bitcoin-qt --version

ninja check
bitcoind --version
bitcoin-cli --version
bitcoin-tx -h
bitcoin-qt --version

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D4177