[Cashtab] [Cashtab] [final bch-api deprecation p2] Change chronikUrl to chronikUrls array

Summary:
T2730, T2735

Depends on D12366

Final stack to get rid of BCH param. In doing this, we will replace Cashtab's method of switching backends.

This diff changes the currency.chronikUrl string to an array of strings, currency.chronikUrls.

Test Plan:
grep -r currency.chronikUrl src/ and confirm it only appears one time in useWallet.js, here changed
npm start and app connects to chronik

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D12788