Set up Oracle JDK 21 for jruby_head workflow

Pin Oracle JDK 21 explicitly via actions/setup-java in the
jruby_head workflow so the Oracle JDBC driver loads against a
known-good runtime instead of whichever JDK the runner image ships
by default. Oracle's ojdbc17.jar is certified with JDK 17 and
JDK 21.

Author: yahonda

.github/workflows/jruby_head.yml

@@ -39,6 +39,11 @@ jobs:
 
     steps:
     - uses: actions/checkout@v6
+    - name: Set up Java
+      uses: actions/setup-java@v4
+      with:
+        distribution: oracle
+        java-version: '21'
     - name: Set up Ruby
       uses: ruby/setup-ruby@v1
       with: