Spring Boot 3: five changes that trip people up when upgrading

1. Java 17 minimum.
2. javax.* became jakarta.*, so imports and some libraries change.
3. Spring Security config moved to the SecurityFilterChain bean style.
4. Observability uses Micrometer Observation API.
5. Native images with GraalVM are first-class.

Upgrade to the latest 2.7 first, fix deprecations, then jump to 3.x.