Add Ruby 3.1 to test matrix

.github/workflows/ci.yml

@@ -11,6 +11,7 @@ jobs:
           - '2.6'
           - '2.7'
           - '3.0'
+          - '3.1'
           - head
           - jruby
           - jruby-head
@@ -26,7 +27,7 @@ jobs:
     - run: bundle exec rake
     # NOTE: calling "bundle install" because the cache apparently did not update due to the gemspec injection
     - run: bundle install && bundle exec standardrb
-      if: matrix.ruby == '3.0' # not using "head" because rubocop won't work there yet
+      if: matrix.ruby == '3.1' # not using "head" because rubocop won't work there yet
   # What's below helps having a single "status check" for mergeability, without
   # having to add each new version to the list of expected status checks in GitHub.
   # See https://github.community/t/status-check-for-a-matrix-jobs/127354/7