Fix build on Gradle 9.5+ and restore publishing on 9.6+

[nedtwigg]

Unblocks #3016 (Gradle 9.4.1 → 9.7.0). Bisecting that PR's failure turned up two independent regressions, not one — and fixing only the first would have converted a loud failure into a silent one.

Breaks in	Symptom	File
9.5.0	Could not get unknown property 'sourceSets' for project ':lib' — hard failure	testlib/build.gradle
9.6.0	publishToMavenLocal silently becomes a no-op — no error at all	gradle/java-publish.gradle

1. Gradle 9.5.0 — project(path).sourceSets

Inside a dependencies { } block, project(path) used to resolve to Project.project(String) and return a Project. As of 9.5.0 it resolves to DependencyHandler.project(String) and returns a ProjectDependency, which has no sourceSets. Same expression, same script:

9.4.1: project(String) -> ...project.LifecycleAwareProject_Decorated
9.5.0: project(String) -> ...dependencies.DefaultProjectDependency_Decorated

The line turns out to be unnecessary. :lib's jar already bundles every glue source set (jar { from sourceSets.getByName(glue).output.classesDirs }), and that jar is on testlib's runtime classpath — which is where FeatureClassLoader resolves com.diffplug.spotless.glue.*. So it's simply dropped.

2. Gradle 9.6.0 — publishing silently stops

Publishing was wrapped in the legacy software-model model { publishing { … } } block. In 9.6.0 that rule no longer binds, so the pluginMaven publication is never created:

9.5.1: > Task :lib-extra:publishPluginMavenPublicationToMavenLocal   (13 tasks executed)
9.6.0: > Task :lib-extra:publishToMavenLocal UP-TO-DATE              (nothing)

Unwrapping to a plain publishing { } fixes it. Ignoring whitespace the diff is exactly two deleted lines. The software model is slated for removal in Gradle 10 regardless, so this was owed anyway.

Neither change is mentioned in Gradle's 9.5/9.6 release notes or the 9.x upgrade guide. Both look worth reporting upstream, especially the silent one.

Verification at 9.7.0

• ./gradlew spotlessCheck — the job that failed on chore(deps): update gradle to v9.7.0 #3016 — passes
• ./gradlew assemble testClasses passes
• ./gradlew build -x spotlessCheck -PSPOTLESS_EXCLUDE_MAVEN=true (CI's main job) — 748 tests, 0 failures
• publishToMavenLocal runs the same 66 tasks as on 9.4.1
• Generated pom-default.xml + module.json are byte-identical before/after for all 4 published projects
• signPluginMavenPublication and publishPluginMavenPublicationToSonatypeRepository (which changelogBump depends on) still exist under -Prelease=true

Everything still passes on 9.4.1, so this lands safely ahead of the wrapper bump — #3016 should go green on rebase without changes of its own.

No CHANGES.md entry: build infrastructure only, nothing user-facing.

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