ci jruby: use Java 21 for jruby

refs: ruby/strscan@baa5de3
refs: ruby/strscan@a4afd32

Author: ydah

.github/workflows/test.yml

@@ -40,6 +40,12 @@ jobs:
     - uses: ruby/setup-ruby@v1
       with:
         ruby-version: ${{matrix.ruby}}
+    - uses: actions/setup-java@v4
+      with:
+        distribution: zulu
+        java-version: 21
+      if: >-
+        startsWith(matrix.ruby, 'jruby')
     - run: bundle install --jobs 4 --retry 3
     - run: rake test
     - run: rake build