test: Remove support for Node 18 (openedx#1454)

* test: Remove support for Node 18

* chore: update code coverage artifact naming

.github/workflows/validate.yml

@@ -9,21 +9,16 @@ on:
 jobs:
   tests:
     runs-on: ubuntu-latest
-    strategy:
-      matrix:
-        node: [18, 20]
     steps:
     - uses: actions/checkout@v4
     - uses: actions/setup-node@v4
       with:
-        node-version: ${{ matrix.node }}
+        node-version-file: '.nvmrc'
     - run: make validate.ci
     - name: Archive code coverage results
       uses: actions/upload-artifact@v4
       with:
-        name: code-coverage-report-${{ matrix.node }}
-        # When we're only using Node 20, replace the line above with the following:
-        # name: code-coverage-report
+        name: code-coverage-report
         path: coverage/*.*
   coverage:
     runs-on: ubuntu-latest
@@ -33,9 +28,7 @@ jobs:
     - name: Download code coverage results
       uses: actions/download-artifact@v4
       with:
-        name: code-coverage-report-20
-        # When we're only using Node 20, replace the line above with the following:
-        # name: code-coverage-report
+        name: code-coverage-report
     - name: Upload coverage
       uses: codecov/codecov-action@v4
       with: