Revert "Bump version to 0.20.0"

Summary:
This reverts commit b6179d8326eb550fba898d7bbbb0aadc0a7a1833.

We don't currently have a way to branch and hotfix the release candidate. Reverting this for now so we can re-tag.

Test Plan:

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

Reviewers: #bitcoin_abc, deadalnix, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D3851