Revert "test: Remove support for Node 18 (#397)"

This reverts commit 6d70b87. Node 18 removal PRs should be merged after Sumac is cut.

Author: brian-smith-tcril

.github/workflows/ci.yml

@@ -3,6 +3,9 @@ on: [push, pull_request]
 jobs:
   tests:
     runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        node: [18, 20]
     steps:
     - name: Checkout
       uses: actions/checkout@v4
@@ -11,7 +14,7 @@ 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

.github/workflows/lockfileversion-check.yml

@@ -10,4 +10,4 @@ on:
 
 jobs:
   version-check:
-    uses: openedx/.github/.github/workflows/lockfileversion-check-v3.yml@master
+    uses: openedx/.github/.github/workflows/lockfile-check-v20.yml@master