build: use Node 20, test on 18 and 20

Author: jrolfs

.github/workflows/build.yml

@@ -29,7 +29,7 @@ jobs:
     if: ${{ needs.deduplicate.outputs.should_skip != 'true' }}
     strategy:
       matrix:
-        node: ['14', '16', '18']
+        node: ['18', '20']
 
     steps:
       - name: Get Yarn cache path

.node-version

@@ -1 +1 @@
-18.18.2
+20.9.0