[chronik-swap] switch default chronik servers

Summary: Switch default chronik for all apps until server is back up

Test Plan:
Confirm this is swapping to 'https://chronik.be.cash/xec'

Keeping it in one diff so easy to revert.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14090