test: Remove support for Node 18 (#987)

.github/workflows/ci.yml

@@ -11,17 +11,14 @@ on:
 jobs:
   tests:
     runs-on: ubuntu-latest
-    strategy:
-      matrix:
-        node: [18, 20]
 
     steps:
     - name: Checkout
       uses: actions/checkout@v3
     - name: Setup Nodejs
       uses: actions/setup-node@v4
       with:
-        node-version: ${{ matrix.node }}
+        node-version-file: '.nvmrc'
 
     - name: Install NPM
       run: npm install