Remove the draft prefix for `ERC20Permit` and `IERC20Permit` as it is now final

[pcaversaccio]

🧐 Motivation

I just realised that EIP-2612 is finally "Final" (PR). Thx for pushing that through @frangio! I quickly opened this issue to track the renaming (in a backward-compatible way of course like for EIP-712; #3621) of draft-ERC20Permit.sol and draft-IERC20Permit.sol to ERC20Permit.sol and IERC20Permit.sol.