Consolidate solidity deps  #388

See https://github.com/primev/mev-commit/blob/main/contracts/remappings.txt, we reference https://github.com/OpenZeppelin/openzeppelin-contracts twice. Once directly from lib/openzeppelin-contracts/ and once as a dep of a dep via lib/openzeppelin-contracts-upgradeable/lib/openzeppelin-contracts/contracts/. This is not good practice as it's unclear which version of the dep we're using in various locations.