test: Remove support for Node 18

Complete the upgrade to Node 20 by removing the Node 18 CI check and
going back to using `.nvmrc` as the source of truth for which version to
use.

.github/workflows/ci.yml

@@ -9,9 +9,6 @@ on:
 jobs:
   test:
     runs-on: ubuntu-latest
-    strategy:
-      matrix:
-        node: [18, 20]
 
     steps:
     - name: Checkout
@@ -21,7 +18,7 @@ jobs:
     - name: Setup Nodejs
       uses: actions/setup-node@v4
       with:
-        node-version: ${{ matrix.node }}
+        node-version-file: '.nvmrc'
     - name: Install dependencies
       run: npm ci
     - name: Lint