Add CI matrix testing and fix dependency scopes

[endrju19]

Summary

• Bump Spring Boot baseline to 3.5.12 (Spring Framework 6.2.17) — lowest actively supported version
• Fix kafka-clients scope from implementation to compileOnly — consumer provides their own version
• Add CI version matrix testing okapi-spring-boot with Spring Boot 3.5/4.0 and okapi-kafka with Kafka 3.9/4.0
• Fix MockProducer compat — updated constructor for Kafka 3.9/4.0 cross-compatibility
• Document supported versions in README and dependency strategy in CLAUDE.md

Verified compatibility

Module	Versions tested
okapi-spring-boot	Spring Boot 3.5.12 + 4.0.4
okapi-kafka	Kafka 3.9.0 + 4.0.2

Test plan

• ./gradlew test — all tests pass with baseline versions
• ./gradlew :okapi-spring-boot:test -PspringBootVersion=4.0.4 -PspringVersion=7.0.6 — passes
• ./gradlew :okapi-kafka:test -PkafkaVersion=4.0.2 — passes
• CI workflow runs on this PR (first run)