Update chainparams for 0.19.6 release

Summary: Per release process. Testnet skipped due to continued reorgs.

Test Plan:

make check

Compare against your own node

Reviewers: #bitcoin_abc, deadalnix, jasonbcox, Mengerian

Reviewed By: #bitcoin_abc, deadalnix, jasonbcox, Mengerian

Subscribers: Mengerian

Differential Revision: https://reviews.bitcoinabc.org/D3051