Gradle v9 support

Hello, I think this project can support gradle v9 but there is an error for now.

  Welcome to Gradle 9.0.0!
  
  Here are the highlights of this release:
   - Configuration Cache is the recommended execution mode
   - Gradle requires JVM 17 or higher to run
   - Build scripts use Kotlin 2.2 and Groovy 4.0
   - Improved Kotlin DSL script compilation avoidance
  
  For more details see https://docs.gradle.org/9.0.0/release-notes.html
  
  To honour the JVM settings for this build a single-use Daemon process will be forked. For more on this, please refer to https://docs.gradle.org/9.0.0/userguide/gradle_daemon.html#sec:disabling_the_daemon in the Gradle documentation.
  Daemon will be stopped at the end of the build 
  
  FAILURE: Build failed with an exception.
  
  * Where:
  Build file '/private/tmp/pdftk-java-20250916-8795-l66o4n/pdftk-v3.3.3/build.gradle' line: 5
  
  * What went wrong:
  An exception occurred applying plugin request [id: 'com.github.johnrengelman.shadow', version: '7.1.2']
  > Failed to apply plugin class 'com.github.jengelman.gradle.plugins.shadow.ShadowJavaPlugin'.
     > Could not get unknown property 'convention' for root project 'pdftk' of type org.gradle.api.Project.
  
  * Try:
  > Run with --stacktrace option to get the stack trace.
  > Run with --info or --debug option to get more log output.
  > Run with --scan to generate a Build Scan (Powered by Develocity).
  > Get more help at https://help.gradle.org.
  

Thanks,