Update testnet documentation

[ardislu]

This PR updates documentation relating to Ethereum's public testnets, especially regarding the Goerli testnet which has been deprecated for almost 2 years now.

Description

• Replace Goerli with Holesky
• Update or remove broken links
• Remove language describing the testnets as newer/older, larger/smaller in size, more/fewer dapps deployed, etc. These facts will change over time, and are therefore more likely to be misleading than useful.

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