[ISSUE #7310] Remove legacy Spring Boot starter metadata - #7398

Summary

Closes #7310.

The main reactor requires Java 17 and pins Spring Boot 3.3.1 (spring-boot.version in the root POM); it declares no older Spring Boot compatibility line for non-client starters. The client starter subtree is excluded from this change so its separate compatibility policy remains intact.

Before removing the legacy files, I compared the 66 non-client spring.factories files (67 registrations) with each module's modern AutoConfiguration.imports; all classes matched exactly and no other factory keys were present. The repeatable check is:

python3 script/verify_spring_boot_auto_configuration.py 08e87ee7a5a80e70edd3a81dcd5ddf2207fef497

It reads the legacy metadata from that migration-base revision and fails if the current imports differ.

This removes the verified non-client spring.factories and spring.provides files, keeps every AutoConfiguration.imports file, and narrows RAT exclusions to the retained client and examples metadata.

Validation

• Gateway, WAF plugin, and Nacos sync-data reactor tests: passed.
• SDK, SDK Feign, registry, and Kubernetes reactor package builds: passed.
• make build-admin build-bootstrap: passed; both binary distributions assembled.
• Started the packaged Admin with local H2 and a temporary JWT key; /actuator/health returned 200 and UP.
• Started the packaged gateway; /actuator/health returned 200 and UP.
• JAR inspection for seven representative starter modules: modern imports present; legacy metadata absent.
• ./mvnw checkstyle:check: passed.
• ./mvnw apache-rat:check: passed.
• Parity script: verified 66 files and 67 registrations.

The checks use the project's configured Spring Boot 3.3.1 baseline; client starters and their metadata were not changed.