test: Remove support for Node 18 (#436)

Co-authored-by: Jason Wesson <jsnwesson@gmail.com>

.github/workflows/ci.yml

@@ -11,9 +11,6 @@ on:
 jobs:
   tests:
     runs-on: ubuntu-latest
-    strategy:
-      matrix:
-        node: [18, 20]
 
     steps:
     - name: Checkout
@@ -22,7 +19,7 @@ jobs:
     - name: Setup Nodejs
       uses: actions/setup-node@v4
       with:
-        node-version: ${{ matrix.node }}
+        node-version-file: '.nvmrc'
 
     - name: Install dependencies
       run: npm ci