CI: Test Node.js versions supported by this repo

This also rewrites our CI config so that all Node.js versions are on one line for ease of future grepping

pelias/pelias#950

Author: orangejulius

.github/workflows/_test.yml

@@ -7,8 +7,7 @@ jobs:
       matrix:
         os:
           - ubuntu-22.04
-        node-version:
-          - 18.x
+        node-version: [ 18.x, 20.x ]
     steps:
       - uses: actions/checkout@v4
       - name: 'Install node.js ${{ matrix.node-version }}'