Fully switch to Java 21 for jruby-head build

Author: headius

.github/workflows/upstream.yml

@@ -182,6 +182,10 @@ jobs:
       - uses: actions/checkout@v4
         with:
           submodules: true
+      - uses: actions/setup-java@v4
+        with:
+          distribution: zulu
+          java-version: 21
       - uses: ruby/setup-ruby@v1
         with:
           ruby-version: "jruby-head"