Updated chainparams for 0.19.8 release

Summary:
Per release process.

Testnet current height: 1308592
Chainparams height selected: 1307745

Test Plan:

make check
# Verify against your own nodes on mainnet and testnet

Reviewers: #bitcoin_abc, deadalnix, Fabien, markblundeberg

Reviewed By: #bitcoin_abc, Fabien, markblundeberg

Subscribers: markblundeberg

Differential Revision: https://reviews.bitcoinabc.org/D3321