CI: Test against Node.js version 18

We will have to upgrade node-postal to 1.20 to support Node.js 20, and better-sqlite3 before we can support Node.js 22.

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

Connects pelias/pelias#950

.github/workflows/_test.yml

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