Attempt to fix maven-javadoc-plugin, and remove redundant org.apache.…

…maven.plugins.

This turned out not to work, but it's probably a good idea, anyway.

To work around the actual problem, I am going to try switching from
JDK11 back to JDK8.

pom.xml

@@ -95,7 +95,6 @@
   <build>
     <plugins>
       <plugin>
-        <groupId>org.apache.maven.plugins</groupId>
         <artifactId>maven-compiler-plugin</artifactId>
         <version>3.8.1</version>
         <configuration>
@@ -114,7 +113,6 @@
         </dependencies>
       </plugin>
       <plugin>
-        <groupId>org.apache.maven.plugins</groupId>
         <artifactId>maven-release-plugin</artifactId>
         <version>2.5.3</version>
         <configuration>
@@ -123,12 +121,14 @@
         </configuration>
       </plugin>
       <plugin>
-        <groupId>org.apache.maven.plugins</groupId>
         <artifactId>maven-jar-plugin</artifactId>
         <version>3.2.0</version>
       </plugin>
 			<plugin>
-				<groupId>org.apache.maven.plugins</groupId>
+				<artifactId>maven-javadoc-plugin</artifactId>
+				<version>3.2.0</version>
+			</plugin>
+			<plugin>
 				<artifactId>maven-site-plugin</artifactId>
 				<version>3.9.1</version>
 			</plugin>