Bump maven-javadoc-plugin to 3.12.0

https://github.com/apache/maven-javadoc-plugin/releases/tag/maven-javadoc-plugin-3.12.0

JIRA: LIGHTY-394
Signed-off-by: Peter Suna <peter.suna@pantheon.tech>
(cherry picked from commit 5e040fb)

Author: PeterSuna

lighty-core/lighty-parent/pom.xml

@@ -171,7 +171,7 @@
                 <plugin>
                     <groupId>org.apache.maven.plugins</groupId>
                     <artifactId>maven-javadoc-plugin</artifactId>
-                    <version>3.11.2</version>
+                    <version>3.12.0</version>
                     <configuration combine.children="append">
                         <!-- Keep things quiet except for warnings/errors -->
                         <quiet>true</quiet>