Makes the L1 starting blocknumber concrete.
See commit message in https://github.com/ethereum-optimism/optimism/pull/8515
for an explanation.

The value in the config comes from here:
https://sepolia.etherscan.io/address/0x84457ca9D0163FbC4bbfe4Dfbb20ba46e48DF254#readProxyContract

The contract address sourced from here:
https://github.com/base-org/contract-deployments/blob/910151eb2eed3542efbb4ccf7e38d1ed3b79d8f5/sepolia/2023-09-26-deploy/deployed/addresses.json#L10C27-L10C69