Bump javadoc from 1.0 to 1.6

Bumps javadoc from 1.0 to 1.6.

---
updated-dependencies:
- dependency-name: org.jenkins-ci.plugins:javadoc
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

pom.xml

@@ -148,7 +148,7 @@
     <dependency>
       <groupId>org.jenkins-ci.plugins</groupId>
       <artifactId>javadoc</artifactId>
-      <version>1.0</version>
+      <version>1.6</version>
       <optional>true</optional>
     </dependency>
     <dependency>