Upgrade to Spring Boot 4 / Spring Framework 7

[jeremyprime]

Summary

Upgrade spring-data-valkey and spring-boot-starter-data-valkey to Spring Boot 4 / Spring Framework 7. Spring Boot 3.5 OSS support ends June 30, 2026.

Key Challenges

1. Spring Boot 4 split the monolithic autoconfigure jar into many different modules, requiring the spring-data-valkey starter to be restructured.
2. spring-data-redis has its own Spring Boot 4 breaking changes that need to be brought into spring-data-valkey.
3. Jackson 3 is now the default, requiring all serializers to be migrated from com.fasterxml.jackson.
4. Spring Framework 7 introduces new null-safety annotations, removes @MockBean, and requires Netty 4.2.
5. Test infrastructure was restructured with new module splits and annotation changes.