spring-boot-migration

# Spring Boot 3 to 4 Migration Migrate in controlled stages. Keep behavior changes separate from the framework upgrade. ## Establish the baseline 1. Upgrade to the latest Boot 3.5 maintenance release. 2. Run unit, slice, integration, startup, and migration tests before changing the major version. 3. Remove Boot 3 deprecations and record all explicitly versioned dependencies. 4. Compare the Boot 3.5 and Boot 4 dependency-management reports. 5. Verify Spring Cloud and other portfolio release-train compatibility independently. ## Prepare for Boot 4 - Require Java 17 or newer; prefer Java 21 for application builds. - Replace `javax.*` remnants with Jakarta APIs before the upgrade. - Remove Undertow assumptions; Boot 4 requires a Servlet 6.1-compatible container. - Inventory Jackson 2 custom modules, serializers, `ObjectMapper` beans, and package imports. - Inventory test annotations, especially `@MockBean`, `@SpyBean`, and implicit MockMvc setup. - Add `spring-boot-properties-migrator` temporarily after changing the Boot version, then remove it. ## Migrate dependencies deliberately Boot 4 is more modular. Prefer dedicated starters over relying on incidental transitive dependencies. Expect dedicated starters for web MVC, security tests, Flyway/Liquibase, and technology-specific tests. Use the classic starters only as a temporary diagnostic bridge, never as the final dependency model. ## Verify the result - Run the application with every supported profile. - Exercise schem...