`javadoc:aggregate` fails with compile errors when multi-module project uses different version of the same dependency

### Affected version

3.12.0

### Bug description

Imagine a multi-module project where different modules use different versions of the same dependency - e.g. connectors to version `1` and `2` of some service.

The `javadoc:aggregate` prunes the classpath supplied to the `javadoc` tool to include only the _latest_ version of a dependency - which can lead to compile errors when referencing something exclusively in the older version.

Start at the javadoc:aggregate goal and trace how it prunes the classpath for the javadoc tool. Reproduce the multi-module case with different versions of one dependency, then verify that aggregation compiles successfully when code references APIs exclusive to the older version.

Written by the indexing model from the issue text.