Update etherscan url of Mantle

We have decided to suspend Mantle's explorer-related services and use Etherscan instead. Please update the corresponding Etherscan browser URL. Thank you! [chains](https://github.com/alloy-rs/chains/tree/v0.2.6)/[src](https://github.com/alloy-rs/chains/tree/v0.2.6/src) /named.rs Mantle => { ("https://api.etherscan.io/v2/api?chainid=5000", **"https://mantlescan.xyz"**) } MantleSepolia => ( "https://api.etherscan.io/v2/api?chainid=5003", **"https://sepolia.mantlescan.xyz/"** )