chore: develop on Node 20, CI with 16.17.x + 18 + 20

This change updates this project's Node versions:

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

- CI should test on Node 20, 18, and 16.17.x.

Node 16.17.x matches the version used by CoMapeo-mobile, which is
nodejs-mobile-react-native@16.17.10. (We can't specify Node 16.17.10
exactly because [it does not exist][0]; [the patch version is specific
to nodejs-mobile-react-native][1].)

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

.github/workflows/node.js.yml

@@ -14,7 +14,7 @@ jobs:
 
     strategy:
       matrix:
-        node-version: [14.x, 16.x]
+        node-version: [16.17.x, 18.x, 20.x]
         # See supported Node.js release schedule at https://nodejs.org/en/about/releases/
 
     steps:

.nvmrc

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