Use Oracle JDK 21 for JRuby CI

Switch the Java setup from Temurin 17 to Oracle JDK 21:
- Oracle JDK matches the distribution most ruby-plsql users run in
  production alongside the Oracle JDBC driver, making the CI
  environment more representative.
- Java 21 is the current LTS, satisfies ojdbc17's Java 17+ requirement,
  and matches what JRuby 10.0.5.0 is compiled against (class file 65).
- Oracle JDK 21 is available at no cost under the NFTC license.

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

Author: yahonda

.github/workflows/jruby_head.yml

@@ -42,8 +42,8 @@ jobs:
     - name: Set up Java
       uses: actions/setup-java@v4
       with:
-        distribution: temurin
-        java-version: '17'
+        distribution: oracle
+        java-version: '21'
     - name: Set up Ruby
       uses: ruby/setup-ruby@v1
       with:

.github/workflows/test.yml

@@ -47,8 +47,8 @@ jobs:
       if: startsWith(matrix.ruby, 'jruby')
       uses: actions/setup-java@v4
       with:
-        distribution: temurin
-        java-version: '17'
+        distribution: oracle
+        java-version: '21'
     - name: Set up Ruby
       uses: ruby/setup-ruby@v1
       with: