Update Ruby version matrix in CI workflow

- Replace Ruby 3.3 and 3.4 with 'head' in the test matrix
- Keep Ruby versions 2.7, 3.0, 3.1, and 3.2 unchanged

Author: cardmagic

.github/workflows/ruby.yml

@@ -22,7 +22,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        ruby-version: ['2.7', '3.0', '3.1', '3.2', '3.3', '3.4']
+        ruby-version: ['2.7', '3.0', '3.1', '3.2', 'head']
 
     steps:
     - uses: actions/checkout@v4