The debt in Token/Token vaults shouldn't increase after maturity

[alcueca]

This doesn't seem to be a risk, but means that Token/Token vaults will never be repaid after maturity. They probably wouldn't, anyway.

They can be cleared by liquidation, at a loss for the liquidator.

The fix would be to replace this line for:

if (uint32(block.timestamp) >= series_.maturity && vault_.ilkId != series_.baseId) {

Unfortunately, the Cauldron cannot be replaced, so the issue is here to stay.