Add Ruby 3.1 and 3.2 and remove Ruby 2.6 from Ruby test matrix

Author: marcoroth

.github/workflows/tests.yml

@@ -39,7 +39,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        ruby-version: [2.6, 2.7, '3.0']
+        ruby-version: [2.7, '3.0', 3.1, 3.2]
 
     steps:
       - uses: actions/checkout@master