Don't run ractor benchmarks on stable ruby

The API has changed between 3.4.7 and HEAD. Until 4.0 is realeased we
shouldn't test 3.4.7

Author: eightbitraptor

.github/workflows/test.yml

@@ -64,7 +64,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        ruby: [ruby, head]
+        ruby: [head]
     if: ${{ github.event_name != 'schedule' || github.repository == 'ruby/ruby-bench' }}
     steps:
       - uses: actions/checkout@v3