Upgrade Gradle wrapper to 9.4.1

[iuliiasobolevska]

Summary

• upgrade the Gradle wrapper from 8.14.3 to 9.4.1
• upgrade Nebula OSS from 11.6.0 to the minimal compatible 12.0.0 release
• retain Kotlin 2.0.21 and disable only its incompatible Kotlin Multiplatform POM rewrite for JVM publications
• adapt TestKit fixtures and task validation to Gradle 9
• build downstream examples with their own wrappers

Root cause

Nebula OSS 11.6.0 crashes during Gradle 9 configuration. After moving to Nebula OSS 12.0.0, Kotlin Gradle plugin 2.0.21 POM rewriting calls a Gradle API removed in Gradle 9. Gradle 9 also removes the TestKit -b and -c options and makes plugin task validation failures fatal.

Related PRs

• #924 contains broader upgrade work, including a Kotlin version update. This PR deliberately decouples the Gradle wrapper and Nebula compatibility changes from the Kotlin upgrade to reduce downstream DGS consumer risk.
• #927 is the automated Dependabot wrapper update. The wrapper-only change cannot pass CI without the manual Nebula, Kotlin POM publishing, TestKit, task-validation, and downstream example-build fixes included here.

Verification

• ./gradlew check publishToMavenLocal --warning-mode all
• Gradle plugin and configuration-cache functional tests
• dgs-examples-java full clean check with Gradle 8.4
• dgs-examples-kotlin full clean check with Gradle 8.14.2
• generated Gradle 9 POM dependencies compared with the Gradle 8.14.3 baseline
• push and pull-request GitHub Actions workflows pass with no excluded test tasks