[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