Bump version numbers to 0.19.3

Summary: See title

Test Plan:
ninja
bitcoind --version
bitcoin-qt --version
make
bitcoind --version
bitcoin-qt --version

Reviewers: deadalnix, Fabien, Mengerian, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Subscribers: schancel

Differential Revision: https://reviews.bitcoinabc.org/D2761