➕ : add spring-boot-starter-validation

From spring-boot 2.3, validation is no longer included in the
web-starter.
So we need to import it manually.

See : https://github.com/spring-projects/spring-boot/wiki/Spring-Boot-2.3-Release-Notes#upgrading-from-spring-boot-22

pom.xml

@@ -137,6 +137,12 @@
             <artifactId>spring-boot-starter-actuator</artifactId>
         </dependency>
 
+        <!-- validation of domain model objects -->
+        <dependency>
+            <groupId>org.springframework.boot</groupId>
+            <artifactId>spring-boot-starter-validation</artifactId>
+        </dependency>
+
         <!-- jackson support for kotlin -->
         <dependency>
             <groupId>com.fasterxml.jackson.module</groupId>