test: Remove support for Node 18 (#735)

.github/workflows/ci.yml

@@ -11,17 +11,14 @@ on:
 jobs:
   tests:
     runs-on: ubuntu-22.04
-    strategy:
-      matrix:
-        node: [18, 20]
     steps:
       - name: Checkout
         uses: actions/checkout@v4
 
       - name: Setup Nodejs
         uses: actions/setup-node@v4
         with:
-          node-version: ${{ matrix.node }}
+          node-version-file: '.nvmrc'
 
       - name: Install dependencies
         run: npm ci