Set up Java 21 for jruby-head builds

These builds compile the extension, so they need to also have the
command line `javac` be Java 21 when building against JRuby 10.

Author: headius

.github/workflows/ubuntu.yml

@@ -29,6 +29,12 @@ jobs:
         git config --global core.eol lf
         git config --global advice.detachedHead 0
     - uses: actions/checkout@v4
+    - uses: actions/setup-java@v4
+      with:
+        distribution: zulu
+        java-version: 21
+      if: >-
+        matrix.ruby == 'jruby-head'
     - name: Set up Ruby
       uses: ruby/setup-ruby@v1
       with: