Upgrade contracts to solidity 0.8.x

[amusingaxl]

Since Maker switched to 0.8.x a while ago and this repo is still under active development, I believe it would be a good thing to update it to 0.8.x to keep it consistent with new additions to the protocol.

Notice that this would require RwaUrn and RwaUrn2 to be audited, since they are authorized on the Vat.