[Cashtab] Bump required node version
[Cashtab] Bump required node version

Summary: Upgraded webpack + upgraded jest supports up to Node 16. Update .nvmrc file to reflect.

Test Plan:
```
nvm use 16
npm test
```

all pass

Reviewers: #bitcoin_abc, kieran709

Reviewed By: #bitcoin_abc, kieran709

Differential Revision: https://reviews.bitcoinabc.org/D11272

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