Update to ChainLink v0.8 and Solidity 0.8

[talentlessguy]

Hello, thanks for this repository, I'm currently learning by it but I found out the project uses a bit old versions of ChainLink and Solidity. I'm writing my own project rn with the latest Solidity compiler and have some issues so would be cool if this repo got updated to latest ChainLink / Solidity / OpenZeppelin

[PatrickAlphaC]

Thanks! Labelled as an enhancement.