Add Node.js 21 to the CI matrix

Author: tnunamak

.github/workflows/nodejs-ci-action.yml

@@ -15,7 +15,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        node-version: [16.x, 18.x, 20.x]
+        node-version: [16.x, 18.x, 20.x, 21.x]
 
     steps:
     - uses: actions/checkout@v2