Migrate a Spring Boot 3.x application to Spring Boot 4.0 (Spring Framework 7, Jackson 3, modular starters, JSpecify). Use only for an explicit Boot 3.x to 4 migration, or to fix build or test failures caused by it, or to review one. Not for 3.x patch bumps or 4.0 to 4.1 upgrades; follow the target Boot version release notes and upgrade guide for those. Covers Maven and Gradle (Kotlin DSL, version
