Add Ruby 3.3 to test matrix (#41)

This PR adds Ruby 3.3 to our CI workflow.

Adding 3.4 is a little more complicated because there aren't any
versions of Nokogiri that support both Ruby 3.0 and Ruby 3.4.

.github/workflows/tests.yml

@@ -12,7 +12,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        ruby: ["3.0", "3.1", "3.2"]
+        ruby: ["3.0", "3.1", "3.2", "3.3"]
         gemfile:
           - gemfiles/rails_6_1.gemfile
           - gemfiles/rails_7_0.gemfile