docs: Java/Kotlin SDK v2.9.3 release notes

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Documents the Java/Kotlin SDK v2.9.3 release.

Changes in v2.9.3:

• Spring AI 2.x compatibility fix: sdk-serde-jackson now uses victools 5.x, resolving a classpath conflict (NoSuchMethodError at startup) for users combining sdk-spring-boot-starter with Spring AI 2.x dependencies.
• Netty version requirement: Due to CVEs, Netty ≥ 4.1.132 is now required. Users on Spring Boot < 3.5.13 need to pin Netty explicitly.

Doc changes:

• Added v2.9.3 entry to the Java/Kotlin SDK changelog
• Added Netty version warning to the Spring Boot docs page
• Bumped SDK version references from 2.9.0 → 2.9.3 in services, tracing, and Spring Boot pages

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