Update test.yml to only use Node.js v20

Author: thadguidry

.github/workflows/test.yml

@@ -8,7 +8,7 @@ jobs:
     name: 'Tests (Node ${{ matrix.node_version }})'
     strategy:
       matrix:
-        node_version: [16, 18]
+        node_version: [20]
     runs-on: 'ubuntu-22.04'
     steps:
       - name: 'Checkout the current branch'
@@ -20,4 +20,4 @@ jobs:
       - name: 'Install Node modules'
         run: 'npm ci'
       - name: 'Run tests'
-        run: 'npm run test'
+        run: 'npm run test'