fix spring-boot-maven-plugin version to be compatible with Java 11

Author: Roman Shulnyaev

metrics-facade-spring/metrics-facade-spring-boot-sample-app/pom.xml

@@ -59,6 +59,7 @@
             <plugin>
                 <groupId>org.springframework.boot</groupId>
                 <artifactId>spring-boot-maven-plugin</artifactId>
+                <version>${spring-boot.version}</version>
 
                 <executions>
                     <execution>