`javadoc:aggregate-jar` is not marked as thread-safe

### Affected version

3.12.0

### Bug description

The [documentation](https://maven.apache.org/plugins/maven-javadoc-plugin/aggregate-jar-mojo.html) says the `javadoc:aggregate-jar` goal isn't thread safe, leading to warnings when executed with a parallel build:
> Warning: *****************************************************************
Warning: * Your build is requesting parallel execution, but this *
Warning: * project contains the following plugin(s) that have goals not *
Warning: * marked as thread-safe to support parallel execution. *
Warning: * While this /may/ work fine, please look for plugin updates *
Warning: * and/or request plugins be made thread-safe. *
Warning: * If reporting an issue, report it against the plugin in *
Warning: * question, not against Apache Maven. *
Warning: *****************************************************************
Warning: The following plugins are not marked as thread-safe in my-project:
Warning: org.apache.maven.plugins:maven-javadoc-plugin:3.12.0
Warning:
Warning: Enable debug to see precisely which goals are not marked as thread-safe.
Warning: *****************************************************************

Other goals _are_ declared thread-safe:
https://github.com/apache/maven-javadoc-plugin/blob/a308a1444b97943fe827befbd0dc25510268fc55/src/main/java/org/apache/maven/plugins/javadoc/JavadocNoForkReport.java#L45

So I suspect that rather than being _not_ thread-safe, this annotation is simply missing from the other goals?

Start by comparing the `javadoc:aggregate-jar` goal with the thread-safe goal declaration in `src/main/java/org/apache/maven/plugins/javadoc/JavadocNoForkReport.java` around line 45. Check how the goal is represented in the generated aggregate-jar documentation and verify the result with the relevant Maven Javadoc Plugin tests or documentation build. Done means the goal's thread-safety status is correctly declared and the parallel-build warning is resolved.

Written by the indexing model from the issue text.