fix: use correct kovan testnet explorer urls
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: BG-30204
Refs: f83b5cd742149f81d2a9a2074f22d8aa812a964c

BitGo/BitGoJS,BitGo/BitGoJS,BitGo/BitGoJS,BitGo/BitGoJS