dep: update minimum java version from 1.8 to 1.9

Github Actions started failing on jruby-head builds this week with an
error message like:

```
  bad class file: /home/runner/.rubies/jruby-head/lib/jruby.jar(/org/jruby/Ruby.class)
    class file has wrong version 65.0, should be 61.0
```

If I understand correctly, the class file version is correlated with
the JDK version, so I'm trying this to see if it fixes the issues with
jruby-head.

See related commit 86652b1 which updated the minimum from 1.7 to 1.8
in 2024, and 08e5560 which updated the minimum from 1.6 to 1.7 in
2020.

Author: flavorjones

rakelib/extensions.rake

@@ -331,8 +331,8 @@ if java?
 
     ext.ext_dir = "ext/java"
     ext.lib_dir = "lib/nokogiri"
-    ext.source_version = "1.8"
-    ext.target_version = "1.8"
+    ext.source_version = "1.9"
+    ext.target_version = "1.9"
     ext.classpath = ext.gem_spec.files.select { |path| File.fnmatch?("**/*.jar", path) }.join(":")
     ext.debug = true if ENV["JAVA_DEBUG"]
   end