Update plugin org.springframework.boot to v4 (#562)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

languages/jvm/java.com/server/spring.io/build.gradle

@@ -10,7 +10,7 @@ plugins {
   // ✅ Spotless
   id 'com.diffplug.spotless' version '8.1.0'
   // ✅ Spring Boot plugin
-  id 'org.springframework.boot' version '3.5.7'
+  id 'org.springframework.boot' version '4.0.0'
   // ✅ Dependency management plugin for Spring libraries
   id 'io.spring.dependency-management' version '1.1.7'
 }

languages/jvm/kotlinlang.org/gradle/wrapper/spring.io/build.gradle.kts

@@ -6,7 +6,7 @@ plugins {
     pmd
     id("com.diffplug.spotless") version "8.1.0"
 
-    id("org.springframework.boot") version "3.5.7"
+    id("org.springframework.boot") version "4.0.0"
     id("io.spring.dependency-management") version "1.1.7"
     kotlin("plugin.spring") version "2.2.21"
 }