Upgrade samples to Spring Boot 2.3.1.RELEASE

Closes gh-1650

build.gradle

@@ -4,7 +4,7 @@ buildscript {
 		snapshotBuild = version.endsWith('SNAPSHOT')
 		milestoneBuild = !(releaseBuild || snapshotBuild)
 
-		springBootVersion = '2.2.7.RELEASE'
+		springBootVersion = '2.3.1.RELEASE'
 	}
 
 	repositories {

spring-session-samples/spring-session-sample-boot-webflux-custom-cookie/src/main/java/sample/SessionController.java

@@ -36,7 +36,7 @@ public class SessionController {
 	@GetMapping("/")
 	public String index(Model model, WebSession webSession) {
 		model.addAttribute("webSession", webSession);
-		return "index";
+		return "home";
 	}
 
 }

spring-session-samples/spring-session-sample-boot-webflux/src/main/java/sample/SessionController.java

@@ -36,7 +36,7 @@ public class SessionController {
 	@GetMapping("/")
 	public String index(Model model, WebSession webSession) {
 		model.addAttribute("webSession", webSession);
-		return "index";
+		return "home";
 	}
 
 	private static final long serialVersionUID = 2878267318695777395L;

spring-session-samples/spring-session-sample-boot-websocket/spring-session-sample-boot-websocket.gradle

@@ -8,6 +8,7 @@ dependencies {
 	compile "org.springframework.boot:spring-boot-starter-data-jpa"
 	compile "org.springframework.boot:spring-boot-starter-data-redis"
 	compile "org.springframework.boot:spring-boot-starter-websocket"
+	compile "org.springframework.boot:spring-boot-starter-validation"
 	compile "org.springframework.boot:spring-boot-devtools"
 	compile "org.springframework:spring-websocket"
 	compile "org.springframework.security:spring-security-messaging"