Update testnets (e.g. Ropsten to Sepolia)

[ElijahLynn]

• Find/replace Ropsten > Sepolia
• Update Ropsten screenshots
• Update Kovan testnet
• Update Rinkeby testnet

Fixes #1033 (eventually once the TLS cert is updated)

[ElijahLynn]

We might want to update all the testnets in this PR as I think Kovan and Rinkeby are deprecated too.

[ElijahLynn]

This screenshot has a cert error (I bypassed the cert block) but a screenshot like this eventually.

We will have to update the language from "Request 1 ether from faucet" to "Request funds" and add a note that it can take up to 60 minutes. I've been waiting 40 minutes.

[ElijahLynn]

Also the Ropsten visiting link is broken because DNS was deleted: https://ropsten.etherscan.io/tx/0x7c7ad5aaea6474adccf6f5c5d6abed11b70a350fbc6f9590109e099568090c57

[ElijahLynn]

In a few seconds, the new transaction will be mined by the Ropsten miners and your MetaMask wallet will show a balance of 1 ETH.

I'm starting to wonder if "the miners" can't mine because of a cert error, as it has been 55 minutes since I requested test ETH.

[ElijahLynn]

Also, after the rainbow cat animation finishes, I don't get a success or confirmation message, it just disappears and I am back to the original form. So, still not sure it submits a request. I just waited the 60 minutes and submitted my second one and no success yet.

[ElijahLynn]

At this point, I'm not even sure I will spend the $57 on this book after I'm done with the free sample as there is too much errata in it. Really which it was updated so I could trust it more, to purchase it.