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