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:

• bug: forge clone returns wrong evm version foundry#8356
• bug(cast run): cast does not select correct hardfork for given transaction foundry#8446