Regression in Gradle plugin v8.3.0: Cannot set the value of task ':integTest:spotlessJava' property 'taskService'

See https://github.com/tbroyer/gradle-incap-helper/actions/runs/23116714774/job/67143450577?pr=53.
```
Caused by: java.lang.IllegalArgumentException: Cannot set the value of task ':integTest:spotlessJava' property 'taskService' of type com.diffplug.gradle.spotless.SpotlessTaskService loaded with InstrumentingVisitableURLClassLoader(ClassLoaderScopeIdentifier.Id{coreAndPlugins:settings[:]:buildSrc[:]:root-project[:]:project-integTest(export)}) using a provider of type com.diffplug.gradle.spotless.SpotlessTaskService loaded with InstrumentingVisitableURLClassLoader(ClassLoaderScopeIdentifier.Id{coreAndPlugins:settings[:]:buildSrc[:]:root-project[:]:project-incap(export)}).
This can be caused by a plugin being applied to two sibling projects and then using a shared build service. To fix this, use `@ServiceReference` or add the problematic plugin with `apply false` to the root build script.
```

This happens in a multi-project build where the root project doesn't have a `build.gradle.kts`, so the spotless plugin isn't applied on the root project; and indeed adding it with `apply false` works around the error.

Either this should be documented as a new requirement, or the plugin should use `@ServiceReference` (if that makes a difference; but then also require Gradle >= 8)