Specify Node version 16 in package.json

It's recommended to set the Node version explicitly¹, otherwise the
build server chooses a default which can change unexpectedly. For
example, this change was prompted because Heroku recently switched from
16 to 18. Auspice doesn't work on 18 yet², so we need to use 16.

¹ https://devcenter.heroku.com/articles/nodejs-support#specifying-a-node-js-version
² nextstrain/auspice#1553

package.json

@@ -3,6 +3,9 @@
   "version": "0.12.0",
   "author": "James Hadfield",
   "license": "AGPL-3.0-only",
+  "engines": {
+    "node": "16.x"
+  },
   "scripts": {
     "modify-auspice-server": "node scripts/modify-auspice-server.js",
     "postinstall": "npm run modify-auspice-server",