Run tests on Ruby 3.3 rather than HEAD

HEAD was Ruby 3.3 (supported). Ruby 3.4 is a long way from ready, so I'm
not going to worry about support for that until we get closer to
release.

.github/workflows/tests.yml

@@ -8,7 +8,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        ruby: ["2.7", "3.0", "3.1", "3.2", "head"]
+        ruby: ["2.7", "3.0", "3.1", "3.2", "3.3"]
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4