Bump version to 0.20.1

Summary:
As per title.

Depends on D3916 for the test plan.

Test Plan:

make check
ninja check

# For both builds, check the versions match:
bitcoind -version
bitcoin-cli -version
bitcoin-tx -h
bitcoin-qt -version

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D3917