Add Sepolia or Holesky testnet as a default testnet in the extension

[michalinacienciala]

Background
The Goerli testnet currently set as a default testnet in the extension (i.e. one that is added to the networks list after toggling on the testnet support in Settings) is planned to become deprecated with the end of year 2023. The testnet that is planned to replace it is called Holešky and it recently started (source). It's not yet fully established (for example, there are no faucets at the moment and most of the projects isn't integrated with it yet). Another available testnet is Sepolia, which will live till the end of 2026.

Testnets Lifecycle:

Read more in Predictable Ethereum Testnet Lifecycle Proposal.

Tasks
Although it is possible to add testnets via custom networks feature, we'll need to replace the Goerli testnet with either Holesky or Sepolia (or both) at some point before end of the year, so that we wouldn't show a deprecated network. Maybe we could do that gradually, i.e first add a new testnet to the list and then remove Goerli when it reaches EOL.

Tasks

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1. Add support for Mainnet Sepolia and Arbitrum Sepolia testnets #3633
   +0

[beemeeupnow]

looking forward to also having Holesky