Update getBlockExplorerUrlForTx to use etherscan-link

`getBlockExplorerUrlForTx` (in `shared/modules/transaction.utils.js`) has its own approach for generating links for custom networks, but a better approach for this is already implemented in the etherscan-links library. We make `getBlockExplorerUrlForTx` less error prone by the appropriate `@metamask/etherscan-link` methods

Start in shared/modules/transaction.utils.js at getBlockExplorerUrlForTx and read the existing custom-network URL logic. Check the available @metamask/etherscan-link methods, then confirm that the function produces the intended explorer links for custom networks without retaining its separate approach.

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