Update documentation for Java 25 (default) and Spring Boot 3.5.9

- Update Java version references: Java 25 (default, Java 21+ compatible)
- Update Spring Boot version references: 3.5.9
- Update dependency version tables to match current versions
- Update prerequisites: minimum Java 21+

Author: ancongui

README.md

@@ -113,7 +113,7 @@ Add the dependency to your `pom.xml`:
 
 ### Prerequisites
 
-- Java 17 or higher
+- Java 21 or higher
 - Spring Boot 3.x
 - Spring WebFlux (reactive web stack)