The Goerli testnet currently used by Threshold/Keep for development purposes is
planned to become deprecated with the end of year 2023. The testnet that was
created to replace it is called
[Holešky](https://github.com/eth-clients/holesky), however it will take some
time until it gets integrated with by some of the projects we rely on.
As a solution, we decided to switch first to another testnet that is currently
live - Sepolia. This testnet's EOL is planned for 2026, which gives us plenty of
time to move to Holešky before Sepolia gets deprecated.
Until Görli is not dead we want to support both testnets.

The Goerli -> Sepolia migration means that also L2 testnet basing on Goerli
(Base Goerli) needs to be migrated to Sepolia-based chain (Arbitrum Sepolia).
At the moment Arbitrum Sepolia is not supported by Wormhole, so deployment of our
contracts on that testnet is yet not possible. But we're already adding changes
that prepare us for the moment when it will be supported.
Once support is confirmed, we'll need to verify if `wormholeChainID` we use in
the config is correct for the testnet.

The Arbitrum Sepolia testnet is not on the list of chains supported by the
`hardhat-verify` plugin
(https://github.com/NomicFoundation/hardhat/blob/main/packages/hardhat-verify/src/internal/chain-config.ts).
We're adding a custom configuration that will make the contracts verification
possible on that testnet (following the
https://hardhat.org/hardhat-runner/plugins/nomicfoundation-hardhat-verify#adding-support-for-other-networks
docs).

Wormhole has added support for Arbitrum Sepolia, Base Sepolia and Optimism
Sepolia testnets. We want to modify the config of our deployment scripts so that
deployment on Sepolia-based L2s would be possible. As the Chain IDs for Wormhole
Sepolia-based networks differ from from IDs for Mainnet and Goerli-based
testnets, we need to set the right value of `...WormholeChainID` constant for
the the right network we're deploying to.

Adding Sepolia-related contracts needed for deployment on `arbitrumSepolia`
network:
* `external/arbitrumSepolia/ArbitrumTokenBridge.json` - address taken from
  https://docs.wormhole.com/wormhole/blockchain-environments/evm#testnet-contracts-sepolia-421614
* `external/arbitrumSepolia/SolanaWormholeGateway.json` - used the same value as
  in `arbitrumGoreli`
* `external/sepolia/TokenBridge.json` - address taken from
  https://docs.wormhole.com/wormhole/blockchain-environments/evm#testnet-contracts-sepolia-11155111

NOT included in this commit (but needed for the deployment):
* `external/arbitrumSepolia/ArbitrumWormholeTBTC.json` - in order to get the
  address we need to perform attestation of the Ethereum Sepolia TBTC token on
  Arbitrum Sepolia (but atm Arbitrum Sepolia is not available on the list of
  target networks on
  https://wormhole-foundation.github.io/example-token-bridge-ui/#/register)

Locally deployed portal was used to register TBTC.

We're adding artifacts for contracts deployed to Arbitrum Sepolia network.

Some dependencies were wrong/missing.