Remove `DOMAIN_SEPARATOR` function from ERC20Permit

[ernestognw]

🧐 Motivation

ERC20Permit exposes a DOMAIN_SEPARATOR function, which is not recommended since EIP712 has native support for ERC-5267

📝 Details

We should add a deprecation notice in DOMAIN_SEPARATOR and schedule it for removal in 6.0

[Amxx]

This function in unfortunatelly part of https://eips.ethereum.org/EIPS/eip-2612. If we remove it, we break strict ERC compatibility.