fix: savers consistent withdraw dust amount

[gomesalexandre]

Description

Fixes a bug spotted while working on #7766

I was unable to broadcast a withdraw Tx, and while debugging things out, noticed that the sum of all my UTXOs was lower than the dust balance value being sent.

The reason behing this is that useSendThorTx() uses default dust amounts, while THORChain savers withdraw confirm step uses upstream dust amount (lower), which meant balance checks weren't working.

Issue (if applicable)

closes N/A

Risk

High Risk PRs Require 2 approvals

What protocols, transaction types, wallets or contract interactions might be affected by this PR?

Low

Testing

• Try to withdraw any UTXO (tested with BTC)
• Ensure THORChain savers withdraw confirm step "Dust Amount" matches the actual on-chain sent value

Engineering

• ^

Operations

• 🏁 My feature is behind a flag and doesn't require operations testing (yet)

• ^

Screenshots (if applicable)