Upgrade the samples to Spring Boot 2.4.x

Closes gh-723

samples/rest-notes-slate/build.gradle

@@ -1,15 +1,9 @@
-buildscript {
-	repositories {
-		mavenCentral()
-	}
-	dependencies {
-		classpath 'org.springframework.boot:spring-boot-gradle-plugin:2.1.9.RELEASE'
-	}
+plugins {
+	id "eclipse"
+	id "java"
+	id "org.springframework.boot" version "2.4.7"
 }
 
-apply plugin: 'java'
-apply plugin: 'org.springframework.boot'
-apply plugin: 'eclipse'
 apply plugin: 'io.spring.dependency-management'
 
 repositories {
@@ -38,6 +32,9 @@ dependencies {
 	runtimeOnly 'org.atteo:evo-inflector:1.2.1'
 
 	testImplementation 'com.jayway.jsonpath:json-path'
+	testImplementation('org.junit.vintage:junit-vintage-engine') {
+		exclude group: 'org.hamcrest', module: 'hamcrest-core'
+	}
 	testImplementation 'org.springframework.boot:spring-boot-starter-test'
 	testImplementation 'org.springframework.restdocs:spring-restdocs-mockmvc'
 }