Upgrade spring-boot to version 4.

backend/build.gradle.kts

@@ -26,7 +26,7 @@ subprojects {
     }
 
     dependencies {
-        implementation(platform("org.springframework.boot:spring-boot-dependencies:3.5.5"))
+        implementation(platform("org.springframework.boot:spring-boot-dependencies:4.1.1"))
         compileOnly("org.projectlombok:lombok:1.18.40")
         annotationProcessor("org.projectlombok:lombok:1.18.40")
         testImplementation("org.assertj:assertj-core:3.27.4")
@@ -43,7 +43,7 @@ subprojects {
 
 dependencyManagement {
     imports {
-        mavenBom("org.springframework.boot:spring-boot-dependencies:3.5.5")
+        mavenBom("org.springframework.boot:spring-boot-dependencies:4.1.1")
     }
 }