Grails 8.x.x: Gradle 9 support

[jamesfredley]

Issue description

Gradle 9 is targeted for a Q2 2025 release.

It will most likely be supported in Grails 8, which will be based on Spring Boot 4.0.x and Spring Framework 7.0.x.

Gradle 9 includes Groovy 4.0.x, currently 4.0.26, as of milestone 9: https://services.gradle.org/distributions/

[jdaugherty]

This requires rewriting the IntegrationTestPlugin to adopt JVM Test Suites