add ruby 3.0 to test matrix

Author: DannyBen

.github/workflows/test.yml

@@ -10,7 +10,7 @@ jobs:
     runs-on: ubuntu-latest
 
     strategy:
-      matrix: { ruby: ['2.4', '2.5', '2.6', '2.7'] }
+      matrix: { ruby: ['2.4', '2.5', '2.6', '2.7', '3.0'] }
 
     steps:
     - name: Checkout code