Change chain to Polygon Amoy

[sebpalluel]

Polygon is going to switch to Amoy (based on Sepolia): https://polygon.technology/blog/introducing-the-amoy-testnet-for-polygon-pos

• See libs/next/chains/src/lib/index.ts for upgrade. Use the new Alchemy API key
• Setup a Cometh app for Amoy and upgrade COMETH_CONNECT_API_KEY
• Setup OpenZeppelin with Amoy (and delete the one that are not used anymore to have only 1 that remains) https://defender.openzeppelin.com/v2/#/manage/relayers
• Update vercel with new env