build: fix missing version warnings (#792)

Author: chingor13

google-http-client-bom/pom.xml

@@ -126,13 +126,17 @@
         </configuration>
       </plugin>
       <plugin>
+        <groupId>org.apache.maven.plugins</groupId>
         <artifactId>maven-javadoc-plugin</artifactId>
+        <version>3.1.1</version>
         <configuration>
           <skip>true</skip>
         </configuration>
       </plugin>
       <plugin>
+        <groupId>org.apache.maven.plugins</groupId>
         <artifactId>maven-site-plugin</artifactId>
+        <version>3.8.2</version>
         <configuration>
           <skip>true</skip>
         </configuration>