Bump node 16; upgrade CircleCI configuration

[allthesignals]

Summary

Bump the node version and also use a newer node image in CircleCI.

Tasks/Bug Numbers

• Fixes Applicant Portal Back End #1109

Technical Explanation

Specifies a node version in a few ways: using .nvmrc and using "engines" key in package.json.

Also bumps the node version in CI.

Note: I'm not sure how this affects the production environment. If we're still using Heroku, we should check that it uses the right buildpack.

Any other info you think would help a reviewer understand this PR?

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[TylerMatteo]

@allthesignals Looks like some more commits have hit develop since you opened this. Can you rebase your branch and push? Aside from that, this looks good to me. Glad to see that this seems to have been pretty straight forward. I'll initiate the merge, just so I can make sure we're traffic managing with those other commits in develop.

We can do some smoke testing once this is merged into develop, I'll let you know if anything comes up. I'm pretty sure Heroku will determine the node version to use based on the new value in .nvmrc so hopefully any problems from that become apparent when we deploy develop.