Validation script should use the OptimismMintableERC20 interface

[hamdiallam]

Surfaced in #915

L2StandardERC20 imported here is an outdated interface. L2 tokens just implementing the IOptimismMintableERC20 interface should be compatible with the validation script without needing to implement the legacy methods as well.

This part just needs to be updated

[fainashalts]

Hi @hamdiallam I see this is a P0 - can you confirm if so?

[fainashalts]

Hi @hamdiallam just following up! Is this a P0 task?