Update Kotlin Samples

Closes gh-19

reactive/webflux/kotlin/hello-security/build.gradle.kts

@@ -1,16 +1,16 @@
 import org.jetbrains.kotlin.gradle.tasks.KotlinCompile
 
 plugins {
-    id("org.springframework.boot") version "2.2.6.RELEASE"
+    id("org.springframework.boot") version "2.5.0"
     id("io.spring.dependency-management") version "1.0.9.RELEASE"
-    kotlin("jvm") version "1.3.71"
-    kotlin("plugin.spring") version "1.3.71"
+    kotlin("jvm") version "1.5.0"
+    kotlin("plugin.spring") version "1.5.0"
 }
 
 repositories {
     mavenCentral()
     maven {
-        url = uri("https://repo.spring.io/libs-snapshot")
+        url = uri("https://repo.spring.io/snapshot")
     }
 }
 

reactive/webflux/kotlin/hello-security/gradle.properties

@@ -1 +1 @@
-spring-security.version=5.4.0.BUILD-SNAPSHOT
+spring-security.version=5.6.0-SNAPSHOT