Update .nvmrc to use "v16"

[JohnAlbin]

Package containing the bug

Not package-specific

Describe the bug

Before we deprecate Node.js v16 with the 2.0.0 release (#530), we should update the .nvmrc file which is currently pinned at v16.19.0.

The latest release of Node.js 16 was v16.20.2 and there's no reason to pin a developer's Node.js at v16.19.0.

Expected behavior

Running nvm use inside our repo, should use whatever version of Node.js v16 the developer has (or ask them to install the latest version.) That command should not force the developer to download and install a slightly-out-of-date version of Node.

Steps to reproduce:

1. Go to the root of the repo
2. Run nvm use