fix missing plugin versions warning

spring-boot-admin-samples/pom.xml

@@ -55,6 +55,20 @@
                 </excludes>
             </resource>
         </resources>
+        <pluginManagement>
+            <plugins>
+                <plugin>
+                    <groupId>org.apache.maven.plugins</groupId>
+                    <artifactId>maven-war-plugin</artifactId>
+                    <version>${maven-war-plugin.version}</version>
+                </plugin>
+                <plugin>
+                    <groupId>org.springframework.boot</groupId>
+                    <artifactId>spring-boot-maven-plugin</artifactId>
+                    <version>${spring-boot.version}</version>
+                </plugin>
+            </plugins>
+        </pluginManagement>
     </build>
     <profiles>
         <profile>

spring-boot-admin-samples/spring-boot-admin-sample-war/pom.xml

@@ -61,7 +61,6 @@
             <plugin>
                 <groupId>org.apache.maven.plugins</groupId>
                 <artifactId>maven-war-plugin</artifactId>
-                <version>${maven-war-plugin.version}</version>
                 <configuration>
                     <failOnMissingWebXml>false</failOnMissingWebXml>
                     <archive>