Pin Oracle JDK 21 for test_11g_ojdbc11 workflow

ojdbc11.jar is certified with JDK 11 and JDK 21 per Oracle's JDBC
downloads page. Explicitly set up Oracle JDK 21 via actions/setup-java
so the workflow runs against a certified combination instead of
whatever JDK the runner image ships by default.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: yahonda

.github/workflows/test_11g_ojdbc11.yml

@@ -43,6 +43,11 @@ 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: