Pin Java 17 Temurin for jruby_head workflow

ojdbc17.jar requires Java 17+. Explicitly set up Temurin 17 via
actions/setup-java so this scheduled JRuby workflow does not silently
depend on whichever JDK the GitHub runner image happens to ship.

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

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: temurin
+        java-version: '17'
     - name: Set up Ruby
       uses: ruby/setup-ruby@v1
       with: