Java migration guide: Spring Boot 4 test code changes - #2734

Adds a new subsection Test Code Changes under the existing Spring Boot 4 section in the CAP Java 4 → 5 migration guide (java/migration.md).

CAP Java 5 mandates Spring Boot 4. Normal application code is rarely affected because it doesn't touch Spring Boot's APIs directly, but test code typically does — Spring Boot 4's new module structure causes package name changes that can break existing test classes (e.g. the test slice annotations like @AutoConfigureMockMvc, @WebMvcTest, @DataJpaTest).

The new subsection:

• explains why test code is more affected than application code,
• links to the Test Code section of the official Spring Boot 4.0 Migration Guide,
• notes that these changes are already covered by the OpenRewrite community recipes and the commercial Moderne platform recipes, and cross-links back to the existing "Automatic CAP Java Migrations with OpenRewrite" section so users can apply both together.