Migrate to Blockscout

The Optimism repo [uses Etherscan as its block explorer](https://github.com/search?q=repo%3Aethereum-optimism%2Foptimism%20etherscan&type=code). While for Layer 1 this is fine, we are migrating our OPM and OP-Sepolia block explorer to Blockscout. Most of the needed changes are just find and replace, but we also need to switch L2 contract verification to use Blockscout. This is fairly straightforward:

This needs fairly minimal changes to the verification script:

```
--verifier blockscout
--verifier-url { https://explorer.optimism.io/api/ | https://testnet-explorer.optimism.io/api }
```

I don't believe you need an API key for blockscout verification.

Search the repository for the Etherscan references and locate the contract verification script. Update the L2 verification paths to use the Blockscout verifier and the OPM and OP-Sepolia API URLs given in the issue, then verify that the affected configurations still work without an API key.

Written by the indexing model from the issue text.