Settle on Solidity version for mainnet release

[izqui]

Solidity 0.8 will be barely a month old when we are ready to deploy the final versions, and it is a version that introduces significant changes that will impact codegen (implicit over/underflow reverts). I am not sure it is worth the risk, unless it has been audited.

Thoughts on what we should start with @sohkai @facuspagnuolo?