chore: add note about `evm_version` being derived from Etherscan and sometimes making the incorrect assumption.

[zerosnacks]

Because Etherscan sometimes returns default evmVersion we automatically derive from the compiler version and make a best-guess assumption. This sometimes leads to issues.

Related: https://github.com/foundry-rs/foundry/pull/8369/files

This would resolve by documenting it as there is no clear solution:

• https://github.com/foundry-rs/foundry/issues/8356
• https://github.com/foundry-rs/foundry/issues/8446