Update to Spring boot 2.3.4, add Maven dependency

'spring-boot-starter-validation'
Signed-off-by: Pekka Helenius <fincer89@hotmail.com>

bookstore/pom.xml

@@ -7,7 +7,7 @@
 	<parent>
 		<groupId>org.springframework.boot</groupId>
 		<artifactId>spring-boot-starter-parent</artifactId>
-		<version>2.2.4.RELEASE</version>
+		<version>2.3.4.RELEASE</version>
 		<relativePath /> <!-- lookup parent from repository -->
 	</parent>
 	<groupId>com.fjordtek.bookstore</groupId>
@@ -42,6 +42,11 @@
 			<scope>test</scope>
 		</dependency>
 
+		<dependency>
+  			<groupId>org.springframework.boot</groupId>
+  			<artifactId>spring-boot-starter-validation</artifactId>
+		</dependency>
+
 		<dependency>
 			<groupId>org.springframework.boot</groupId>
 			<artifactId>spring-boot-starter-data-jpa</artifactId>
@@ -51,18 +56,7 @@
 			<groupId>com.h2database</groupId>
 			<artifactId>h2</artifactId>
 		</dependency>
-<!-- 
-NOTE: Do NOT enable validation dependency separately because it likely will conflict
-with <parent> version number and therefore, all validation check functionality
-is ignored by Spring Framework.
-Instead, drop down the version number so that we don't need to separately enable
-this dependency. (Find a better solution if needed)
 
-		<dependency>
-			<groupId>javax.validation</groupId>
-			<artifactId>validation-api</artifactId>
-		</dependency>
--->
 	</dependencies>
 
 	<build>