test: Remove support for Node 18 (#163)

.github/workflows/ci.yml

@@ -9,20 +9,15 @@ on:
 jobs:
   tests:
     runs-on: ubuntu-latest
-    strategy:
-      matrix:
-        node: [18, 20]
     steps:
     - name: Checkout
       uses: actions/checkout@v4
       with:
         fetch-depth: 0
-    - name: Setup Nodejs Env
-      run: echo "NODE_VER=`cat .nvmrc`" >> $GITHUB_ENV
     - name: Setup Nodejs
       uses: actions/setup-node@v4
       with:
-        node-version: ${{ matrix.node }}
+        node-version-file: '.nvmrc'
     - name: Install dependencies
       run: npm ci
     - name: Lint