Gradle 9.0 preparation

[andsel]

Currently Logstash uses the Gradle 8.7, there are some deprecations happening in Gradle9.0 that we should fix to be ready for that release.

Running

./gradlew test --warning-mode all

hit 2 warning:

 % ./gradlew test --warning-mode all
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/8.7/userguide/gradle_daemon.html#sec:disabling_the_daemon 
in the Gradle documentation.
Daemon will be stopped at the end of the build
Relying on the convention for Test.classpath in custom Test tasks has been deprecated. 
This is scheduled to be removed in Gradle 9.0. Consult the upgrading guide for further information: 
https://docs.gradle.org/8.7/userguide/upgrading_version_8.html#test_task_default_classpath

...

> Task :logstash-core:javaTests
Relying on the convention for Test.testClassesDirs in custom Test tasks has been deprecated. 
This is scheduled to be removed in Gradle 9.0. Consult the upgrading guide for further information: 
https://docs.gradle.org/8.7/userguide/upgrading_version_8.html#test_task_default_classpath