chore: develop on Node 20, CI with 16.17.10 + 18 + 20

This change updates this project's Node versions:

- Develop on Node 20 by updating `.nvmrc`.
- CI should test on 16.17.10 (the version used by CoMapeo-mobile), 18.x,
  and 20.x.

.github/workflows/node.js.yml

@@ -14,7 +14,7 @@ jobs:
 
     strategy:
       matrix:
-        node-version: [14.x, 16.x]
+        node-version: [16.17.10, 18.x, 20.x]
         # See supported Node.js release schedule at https://nodejs.org/en/about/releases/
 
     steps:

.nvmrc

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