Bump version number to 0.19.8

Summary: As per title.

Test Plan:

bitcoind -version
bitcoind -help
bitcoin-qt -version
bitcoin-qt -help

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, deadalnix, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D3174