build: use Kotlin DSL

vul/vul-springboot3-webflux/build.gradle.kts

@@ -0,0 +1,15 @@
+plugins {
+    id("java")
+    id("org.springframework.boot") version "3.3.7"
+    id("io.spring.dependency-management") version "1.1.7"
+}
+
+java {
+    toolchain {
+        languageVersion = JavaLanguageVersion.of(17)
+    }
+}
+
+dependencies {
+    implementation("org.springframework.boot:spring-boot-starter-webflux")
+}