change references from Goerli to Sepolia in relevant lesson when NFT compatibility arrives

[elPiablo]

We had wanted to use the Sepolia testnet as our go-to testnet in our lessons on creating NFT contracts, because Goerli is going to be deprecated by the end of 2023, but were restricted, because there isn't yet a compatible NFT test hosting site for Sepolia.

Let's update the lessons accordingly when this availability arrives.

• [x] lesson 2
• [x] lesson 3
• [x] lesson 4 if applicable, otherwise tick the box
• [ ] lesson 5 if applicable, otherwise tick the box