[cashtab-faucet, cashtab] Use recaptcha v2 instead of v3 for harder challenges

Summary:
The v3 42-xec reward is constantly being drained by a single attacker. Implement v2 (challenge based) to see if this improves things.

The ultimate goal is to have a sybil-resistant reward system so that rewards can be increased. We may need to go to something like twitter auth or similar to really achieve this.

Test Plan: npm test, I tested this locally with a new v2 key, will update CI before landing

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18951