Fix Java version mismatch errors

We are trying to compile for Java 8 with `-target 1.8 -source 1.8 `at compile time
but the JRuby jar file (jruby.jar) is compiled with Java 21

- Error message: `class file has wrong version 65.0, should be 61.0`
- version 65.0 = Java 21
- version 61.0 = Java 17

Author: ydah

Rakefile

@@ -67,9 +67,31 @@ if jruby?
     jruby_home = RbConfig::CONFIG['prefix']
     lib_dir = ext.lib_dir += "/#{ext.platform}/racc"
     ext.ext_dir = 'ext/racc'
-    # source/target jvm
-    ext.source_version = '1.8'
-    ext.target_version = '1.8'
+
+    # Determine appropriate Java version based on JRuby version
+    # For JRuby 10.x, we compile with Java 21 but target Java 8 bytecode for compatibility
+    # This allows using newer Java while maintaining backward compatibility
+    if defined?(JRUBY_VERSION)
+      case JRUBY_VERSION
+      when /^10\./
+        # JRuby 10.x: compile with available Java but target Java 8 for compatibility
+        ext.source_version = '1.8'
+        ext.target_version = '1.8'
+      when /^9\.4\./
+        # JRuby 9.4.x: use Java 11
+        ext.source_version = '11'
+        ext.target_version = '11'
+      else
+        # JRuby 9.3.x and earlier: use Java 8
+        ext.source_version = '1.8'
+        ext.target_version = '1.8'
+      end
+    else
+      # Default to Java 8 if JRUBY_VERSION is not defined
+      ext.source_version = '1.8'
+      ext.target_version = '1.8'
+    end
+
     jars = ["#{jruby_home}/lib/jruby.jar"] + FileList['lib/*.jar']
     ext.classpath = jars.map { |x| File.expand_path x }.join( ':' )
     ext.name = 'cparse-jruby'