Bump JUnit to JUnit 6+

apple/app-store-server-library-java#279 mergedbuild-only

What it touches

build: 1

Modules: build.gradle (1)

Architectural impact (arcade-agent diff_impact)

No production Java file of this PR maps into the dependency graph (tests, build, CI or docs only). Tests are excluded from the graph by design.

Build & CI relevance

• Touches Gradle/build files: review for build-time or cache impact.

Files