fix spring boot maven plugin version warning

Author: csviri

samples/basic/spring-boot/pom.xml

@@ -55,6 +55,7 @@
             <plugin>
                 <groupId>org.springframework.boot</groupId>
                 <artifactId>spring-boot-maven-plugin</artifactId>
+                <version>2.2.6.RELEASE</version>
             </plugin>
         </plugins>
     </build>