chore: redeploy TierNFT contract and change testnet references. Mumbai is deprecating

[elPiablo]

Mumbai is being retired. TierNFT lesson and Connect to Front-end lessons need upgraded:

https://polygon.technology/blog/polygon-pos-is-cooking-the-napoli-upgrade-means-better-ux-the-mumbai-testnet-takes-a-bow

• remove reference to "scripts/ where simple automation scripts go"

• change to rm contracts/*.sol
rm test/*.js
  by removing rm scripts/*.js as it doesn't come with the build anymore

• change hardhat.config.js with new RPC details: https://polygon.technology/blog/polygon-pos-and-polygon-zkevm-new-testnets-for-polygon-protocols for Amoy Testnet

• update "Adding the testnet network paragraph with new details 👆🏼

• update screenshot from Mumbai to Amoy

• check if URL for test token faucet is correct - add alternatives or additional URL possibilites

• update screenshot re: faucet if necessary

• under Personal Security paragraph, format the current paragraph from text to a bulleted list for Zerion steps

• change deploy command to npx hardhat run scripts/deploy.js --network amoy

• change mint command to npx hardhat run scripts/mint.js --network amoy