[kotlin-client] Update Gradle and dependencies (#18571)

* [kotlin] Target correct library in jvm-spring-webclient sample

* [kotlin] Fixed warning in jvm-spring-restclient

* [kotlin-client] Bump Gradle version

* [kotlin-client] enableFeaturePreview no longer needed as it's enabled by default

* [kotlin-client] Bump kotlin, spotless, and reactor versions

* [kotlin-client] Generated code

* [kotlin-client] Missed a generated sample

* [kotlin-client] Bumped gradle and java version in kotlin-client workflows

* [kotlin-client] First attempt to fix jvm-volley

* [kotlin-client] Use standard gradle action instead of custom one

* [kotlin-client] Use original gradlew action but without specific version

* [kotlin-client] Moved sample kotlin-spring-cloud to servers instead of clients

* [kotlin-client] Added previously missing generated file

* [kotlin-client] Corrected sample path

samples/client/echo_api/kotlin-jvm-spring-3-webclient/build.gradle

@@ -2,16 +2,14 @@ group 'org.openapitools'
 version '1.0.0'
 
 wrapper {
-    gradleVersion = '7.5'
+    gradleVersion = '8.7'
     distributionUrl = "https://services.gradle.org/distributions/gradle-$gradleVersion-all.zip"
 }
 
 buildscript {
-    ext.kotlin_version = '1.8.10'
-    ext.spring_boot_version = "3.2.4"
-    ext.reactor_version = "3.6.4"
-    // 6.13.0 is the latest stable release that supports JDK8
-    ext.spotless_version = "6.13.0"
+    ext.kotlin_version = '1.9.23'
+    ext.spring_boot_version = "3.2.5"
+    ext.spotless_version = "6.25.0"
 
     repositories {
         maven { url "https://repo1.maven.org/maven2" }
@@ -67,6 +65,6 @@ dependencies {
     implementation "com.fasterxml.jackson.module:jackson-module-kotlin:2.17.0"
     implementation "com.fasterxml.jackson.datatype:jackson-datatype-jsr310:2.17.0"
     implementation "org.springframework.boot:spring-boot-starter-webflux:$spring_boot_version"
-    implementation "io.projectreactor:reactor-core:$reactor_version"
+    implementation "io.projectreactor:reactor-core:3.6.5"
     testImplementation "io.kotlintest:kotlintest-runner-junit5:3.4.2"
 }