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
(Mumbai) needs to be migrated to Sepolia-based chain. At the moment the new
testnet isn't yet announced, so deployment of our contract is yet not possible.
But we're already starting to add changes that prepare us for the moment when it
will be supported.
Once new testnet is live and WOrmhole support is confirmed, we'll need to verify
if `wormholeChainID` we use in the config is correct for the testnet. We'll also
need to update the missing Hardhat configuration (TODOs) and add the
`export-artifacts:sepolia` spript to `package.json`.

It was unclear which testnet will replace the Mumbai testnet after the
deprecation of the underlying Goerli testnet. Now we know the new L2 testnet on
Polygon will be called `amoy` and will use the `80002` chain id. We can update
our config accordingly.
Read more:
maticnetwork/heimdall#1096
maticnetwork/bor#1072

Adding Sepolia-related contracts needed for deployment on `amoy` network:
* `external/baseSepolia/BaseTokenBridge.json` - address taken from
  https://docs.wormhole.com/wormhole/blockchain-environments/evm#testnet-contracts-base-sepolia-84532
* `external/amoy/SolanaWormholeGateway.json` - used the same value as in `mumbai`
* `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/amoy/PolygonWormholeTBTC.json` - in order to get the address we need
  to perform attestation of the Ethereum Sepolia TBTC token on Amoy (but atm
  Amoy is not available on the list of target networks on
  https://wormhole-foundation.github.io/example-token-bridge-ui/#/register)
* `external/amoy/PolygonTokenBridge.json` - address needs to be taken from
  https://docs.wormhole.com/wormhole/blockchain-environments/evm once Wormhole
  adds a support for Amoy