[Cashtab] Specify latest stable version of nodejs

Summary: Node 16 is well past end of life. Specify latest stable version of node in Cashtab nvmrc file. Update CONTRIBUTING.md

Test Plan: npm test

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D15367