Update JUnit 5 sample to use Gradle's native JUnit 5 support

See gh-576

samples/junit5/build.gradle

@@ -4,7 +4,6 @@ buildscript {
 	}
 	dependencies {
 		classpath 'org.springframework.boot:spring-boot-gradle-plugin:2.1.3.RELEASE'
-		classpath 'org.junit.platform:junit-platform-gradle-plugin:1.0.1'
 	}
 }
 
@@ -16,7 +15,6 @@ apply plugin: 'java'
 apply plugin: 'org.springframework.boot'
 apply plugin: 'eclipse'
 apply plugin: 'io.spring.dependency-management'
-apply plugin: 'org.junit.platform.gradle.plugin'
 
 repositories {
 	mavenLocal()
@@ -50,6 +48,7 @@ dependencies {
 
 test {
 	outputs.dir snippetsDir
+	useJUnitPlatform()
 }
 
 asciidoctor {