Update maven-pmd-plugin version in Documentation.

[georgehipp]

maven-pmd-plugin version 3.0.1 does not support JDK 1.8:
Failed to execute goal org.apache.maven.plugins:maven-pmd-plugin:3.0.1:pmd (default-cli) An error has occurred in PMD Report report generation.: Unsupported targetJdk value '1.8'.

maven-pmd-plugin Latest Version does not support the current ruleset:
Failed to execute goal org.apache.maven.plugins:maven-pmd-plugin:3.12.0:pmd (default-cli) Execution default-cli of goal org.apache.maven.plugins:maven-pmd-plugin:3.12.0:pmd failed: Unable to exclude rules [UselessParentheses]; perhaps the rule name is mispelled?

maven-pmd-plugin version 3.6 supports JDK 1.8 and allows the current ruleset.

This change will allow users to use the Adobe recommended Java 8 and not fail due to the outdated ruleset.

Future changes should be to the ruleset, ensuring the latest version of PMD and the maven-pmd-plugin is supported.

Validated with
Versions:
Apache Maven 3.5.0
Java version: 1.8.0_202, vendor: Oracle Corporation
Default locale: en_US, platform encoding: UTF-8
OS name: "mac os x", version: "10.13.6", arch: "x86_64", family: "mac"

Command:
mvn pmd:pmd -Panalysis

Plugin Configs:

    <profile>
      <id>analysis</id>
          ...
          <plugin>
            <artifactId>maven-pmd-plugin</artifactId>
            <version>[3.0.1 | 3.6 | 3.12.0]</version>
            <executions>
              <execution>
                <phase>package</phase>
                <goals>
                  <goal>pmd</goal>
                  <goal>cpd</goal>
                  <!-- build will fail on warnings -->
                  <goal>check</goal>
                </goals>
              </execution>
            </executions>
          </plugin>
           ...
    </profile>

If a different JDK version is needed, <targetJdk>[1.6 | 1.7 | etc]</targetJdk> can be used.

Example:

        <configuration>
          <targetJdk>1.6</targetJdk>
        </configuration>

Link: https://maven.apache.org/plugins/maven-pmd-plugin/examples/targetJdk.html#