chore: develop on Node 20, CI with 16.17.1 + 18.17.1 + 20 (#35)

This change updates this project's Node versions:

- Develop on Node 20 by updating `.nvmrc`.

- CI should test on Node 20, 18.17.1, and 16.17.1.

Node 16.17.1 and 18.17.1 match versions of nodejs-mobile-react-native,
used by CoMapeo-mobile. nodejs-mobile has the same major & minor
versions as Node, but [different patch versions][0]. For example,
nodejs-mobile-react-native@16.17.10 corresponds to Node 16.17.1.

[0]: https://github.com/nodejs-mobile/nodejs-mobile-react-native/tree/51afe46d36d3776d1bfe7158073897216b55d7f2#versioning

Co-authored-by: Gregor MacLennan <gmaclennan@digital-democracy.org>
BREAKING CHANGE: Node 16.17.1+ is now required.

.github/workflows/node.js.yml

@@ -14,7 +14,7 @@ jobs:
 
     strategy:
       matrix:
-        node-version: [14.x, 16.x]
+        node-version: [16.17.1, 18.17.1, 20.x]
         # See supported Node.js release schedule at https://nodejs.org/en/about/releases/
 
     steps:

.nvmrc

@@ -1 +1 @@
-14
+20