Upgrade Node to latest

[rfultz]

Summary

We use Node to build our site.
We're currently using Node 10.16.0 (and npm 6.10.2).
According to the release schedule:

• Node 10.16.0 is currently in maintenance mode and will reach its end of life (no more updates) in a few months (30 April 2021)
• Node 12 is active but will enter maintenance mode next month (30 November 2020) and end of life in April 2023
• Node 14 is current and will start LTS (long-term support) tomorrow (27 October 2020)
• Node 16 isn't expected until April 2021
• Node isn't a fan of odd-numbered releases

Because

• our current version is nearly out of date,
• the next version is current,
• but the next-next version will be current probably by the time you read this, and
• because our current CircleCI image defaults to a newer version of Node and we specifically roll it back,

I propose we upgrade from 10 to 14 (skipping 12).

Upgrading should keep us up to date with capabilities and reported vulnerabilities