Update chain parameters for 0.18.8

Summary:
Main chain parameters updated to a recent block.
Testnet left unchanged, as the default assume valid block is
within 10,000 block of the chain tip.

Test Plan:
Run bitcoin-qt and check that the block hash is in the chain.
Also check that work done is correct.

Reviewers: deadalnix, jasonbcox, #bitcoin_abc

Reviewed By: jasonbcox, #bitcoin_abc

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2510