[ci] add the "current" Node.js version to matrix testing

Currently the current release is Node.js 21.x

.github/workflows/ci.yml

@@ -15,7 +15,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        node: [18, 20]
+        node: [18, 20, current]
     steps:
       - uses: actions/checkout@v4
       - name: Setup node