Update chainparams for 0.18.6 release

Summary: Update chainparams using recent block on mainnet and a high-difficulty block, 2000 blocks in the past on testnet.

Test Plan: ninja

Reviewers: deadalnix, Fabien, Mengerian, #bitcoin_abc

Reviewed By: Mengerian, #bitcoin_abc

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2270