Update the block that's assumed to be valid to be post fork for both main and testnet
Update the block that's assumed to be valid to be post fork for both main and testnet

Summary: As per title.

Test Plan: Reindex testnet and mainnet.

Reviewers: freetrader, CCulianu, #bitcoin_abc

Reviewed By: CCulianu, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D418

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