Remove Ruby 2.6 and 2.7 from test matrix and add 3.3

.github/workflows/unit-tests.yml

@@ -10,7 +10,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        ruby: ["2.6", "2.7", "3.0", "3.1", "3.2"]
+        ruby: ["3.0", "3.1", "3.2", "3.3"]
     steps:
       - name: Checkout repository
         uses: actions/checkout@v3