refresh the mainnet.json used for blockchain util tests

[macfarla]

PR description

Think it makes sense for this (test) genesis file to be fairly up to date. Used by BlockchainSetupUtil

Fixed Issue(s)

related to #8944

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• Checked out our contribution guidelines?
• Considered documentation and added the doc-change-required label to this PR if updates are required.
• Considered the changelog and included an update if required.
• For database changes (e.g. KeyValueSegmentIdentifier) considered compatibility and performed forwards and backwards compatibility tests

Locally, you can run these tests to catch failures early:

• spotless: ./gradlew spotlessApply
• unit tests: ./gradlew build
• acceptance tests: ./gradlew acceptanceTest
• integration tests: ./gradlew integrationTest
• reference tests: ./gradlew ethereum:referenceTests:referenceTests

[Gabriel-Trintinalia]

LGTM