message: fix: use correct kovan testnet explorer urls
  Kovan moved the API endpoints to the `api-kovan` subdomain, but the
  block explorer UI is still on `kovan`.
  Ticket:
  Refs:
diff: @@ -228,8 +228,8 @@ class Pyrmont extends Testnet implements AccountNetwork {
  class Kovan extends Testnet implements AccountNetwork {
  family = CoinFamily.ETH;
  - explorerUrl = 'https://api-kovan.etherscan.io/tx/';
  - accountExplorerUrl = 'https://api-kovan.etherscan.io/address/';
  + explorerUrl = 'https://kovan.etherscan.io/tx/';
  + accountExplorerUrl = 'https://kovan.etherscan.io/address/';
  }
  class Goerli extends Testnet implements AccountNetwork {
  