[MNG-11642] Use legacyMode for aggregate javadoc with mixed JPMS modules

The aggregate javadoc report fails when some modules have module-info.java
(named modules) and others don't (unnamed modules). Using legacyMode forces
classpath-based javadoc generation, bypassing the named/unnamed conflict.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: gnodet

pom.xml

@@ -1126,6 +1126,7 @@ under the License.
             <groupId>org.apache.maven.plugins</groupId>
             <artifactId>maven-javadoc-plugin</artifactId>
             <configuration>
+              <legacyMode>true</legacyMode>
               <sourceFileExcludes>
                 <sourceFileExclude>module-info.java</sourceFileExclude>
               </sourceFileExcludes>