upgraded Spring Boot version from 4.0.5 to 4.0.6

backend/build.gradle.kts

@@ -1,6 +1,6 @@
 plugins {
 	java
-	id("org.springframework.boot") version "4.0.5"
+	id("org.springframework.boot") version "4.0.6"
 	id("io.spring.dependency-management") version "1.1.7"
 }