chore: update fx rates endpoint

[pwltr]

Description

Update FX rates endpoint to no longer use old Blocktank service.

See https://synonymworkspace.slack.com/archives/C07BJ7DNPCG/p1784012810894369

[greptile-apps]

Greptile Summary

This PR moves the production FX rates request off the old Blocktank service.

• Updates the Bitcoin btcRatesServer URL.
• Leaves signet, testnet, and regtest rates URLs unchanged.

Confidence Score: 5/5

This looks safe to merge after confirming the new rates endpoint returns the same public JSON contract.

• The code change is limited to one production URL constant.
• The existing caller expects an unauthenticated JSON response with a top-level tickers array.
• No blocking issue was found in the changed code.

Bitkit/Constants/Env.swift

Important Files Changed

Filename	Overview
Bitkit/Constants/Env.swift	Changes the production FX rates endpoint used by currency rate fetching.

_{Reviews (1): Last reviewed commit: "docs: update changelog" | Re-trigger Greptile}

[jvsena42]

The url return matches the current model

[jvsena42]

The url response matches the project model