Revert "test: Remove support for Node 18 (openedx#736)" (openedx#740)

This reverts commit dc5f097. Node 18 removal PRs should be merged after Sumac is cut.

.github/workflows/ci.yml

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