[Cashtab + docs] Bump node version

Summary: Resolve build + deploy issue caused by docs upgrade requiring node v16

Test Plan:

cd web/cashtab
docker build -t cashtab_local .
docker run --rm -p 8080:80 --name cashtab cashtab_local

confirm the app still works

same for docs/

Reviewers: #bitcoin_abc, kieran709

Reviewed By: #bitcoin_abc, kieran709

Differential Revision: https://reviews.bitcoinabc.org/D12011