Set default gasLimit to 30M for mainnet

[jmcpheron]

Description

Mainnet miners have voted to keep the gasLimit around 30,000,000 since August 2021. Geth had an 8,000,000 gasLimit default on a merge testnet which resulted in a drop in the gasLimit.

Post-merge lots of validators will run with default configs, which means the gas limit will move to the default gas limit.
On mainnet-shadow-fork-1 this resulted in a drop from a gaslimit of 30M down to 8M.

Geth has since updated their default to 30M.

If I an understanding the Besu default mainnet setting correctly the default gasLimit is still 5000 (The original limit at genesis 😲). I think this should be incresed to 30M in prepeartion for The Merge since there is a chance that validators may not understand/care about miner setting.

[non-fungible-nelson]

closing in favor of #3768