Fix JRuby CI for Windows (#151)

The setup-ruby logic for JRuby on Windows must not be fully choosing
Java 21, so when it tries to compile the extension it's still using an
older version. This fixes it.

Author: headius

.github/workflows/ci.yml

@@ -45,7 +45,7 @@ jobs:
           distribution: zulu
           java-version: 21
         if: >-
-          matrix.ruby == 'jruby-head'
+          startsWith(matrix.ruby, 'jruby')
 
       - name: Set up Ruby
         uses: ruby/setup-ruby@v1