fix(utxo-lib): default to `version: 2` for BTG transactions
fix(utxo-lib): default to `version: 2` for BTG transactions

We do this in the SDK so why not do it in utxo-lib already.

The meaning of the version field is explained in BIP0068:
https://github.com/bitcoin/bips/blob/master/bip-0068.mediawiki#Abstract

Issue: BG_34381

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