Add Ruby 3.3 to CI matrix

Author: m-nakamura145

.github/workflows/tests.yml

@@ -5,7 +5,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        ruby: ['2.6', '2.7', '3.0', '3.1', '3.2', 'head']
+        ruby: ['2.6', '2.7', '3.0', '3.1', '3.2', '3.3', 'head']
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v2