CI: give the JRuby leg Java 21

JRuby 10.x is compiled for Java 21, but the runner defaults to Java 17,
so the jruby leg failed with UnsupportedClassVersionError before it could
even start. Add a conditional actions/setup-java@v4 step (Temurin 21) for
matrix entries that set java-version, and set java-version: "21" on jruby.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: trans

.github/workflows/ci.yml

@@ -4,41 +4,46 @@ on: [push, pull_request]
 
 jobs:
   test:
-    name: Ruby ${{matrix.ruby}} on ${{matrix.os}}
-    runs-on: ${{matrix.os}}-latest
-    continue-on-error: ${{matrix.experimental}}
+    name: Ruby ${{ matrix.ruby }} on ${{ matrix.os }}
+    runs-on: ${{ matrix.os }}-latest
+    continue-on-error: ${{ matrix.experimental }}
 
     strategy:
       matrix:
         os:
           - ubuntu
-
         ruby:
           - "3.1"
           - "3.2"
           - "3.3"
           - "3.4"
-
         experimental: [false]
-
         include:
           - os: ubuntu
             ruby: truffleruby
             experimental: true
           - os: ubuntu
             ruby: jruby
             experimental: true
+            java-version: "21"
           - os: ubuntu
             ruby: head
             experimental: true
 
     steps:
-    - uses: actions/checkout@v4
-    - uses: ruby/setup-ruby@v1
-      with:
-        ruby-version: ${{matrix.ruby}}
-        bundler-cache: true
-
-    - name: Run tests
-      timeout-minutes: 5
-      run: bundle exec rake test
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-java@v4
+        if: ${{ matrix.java-version }}
+        with:
+          distribution: temurin
+          java-version: ${{ matrix.java-version }}
+
+      - uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: ${{ matrix.ruby }}
+          bundler-cache: true
+
+      - name: Run tests
+        timeout-minutes: 5
+        run: bundle exec rake test