Fix: Mainnet fork not working due to outdated hardhat

[Turupawn]

npx hardhat node errors while trying to fork mainnet on local hardhat the following way:

    networks: {
        hardhat: {
            forking: {
              url: MAINNET_RPC_URL
            }
        },

This is because older versions of Hardhat are not compatible with london hardfork. Luckly, Hardhat 2.6.4 is compatible so upgrading fixes this issue.

[PatrickAlphaC]

Thanks for this PR! I added some stuff cleaning up yarn and package.lock as well. Thanks!

[Turupawn]

Ah right I also had trouble with Polygon and Mumbai on the truffle config. Thanks for fixing that.

Cheers!