Java: Set javadoc to legacymode (#3041)

This will avoid an error:

```
[INFO] Reactor Summary for Apache Avro Java 1.12.0-SNAPSHOT:
[INFO]
[INFO] Apache Avro Java ................................... SUCCESS [  2.043 s]
[INFO] Apache Avro ........................................ SUCCESS [  8.623 s]
[INFO] Apache Avro Android Compatibility .................. SUCCESS [  1.571 s]
[INFO] Apache Avro IDL .................................... SUCCESS [  5.358 s]
[INFO] Apache Avro Compiler ............................... SUCCESS [  3.918 s]
[INFO] Apache Avro Maven Plugin ........................... SUCCESS [  4.313 s]
[INFO] Apache Avro IPC .................................... SUCCESS [  5.228 s]
[INFO] Apache Avro IPC Jetty .............................. SUCCESS [  2.087 s]
[INFO] Apache Avro IPC Netty .............................. SUCCESS [  2.386 s]
[INFO] Trevni Java ........................................ SUCCESS [  0.132 s]
[INFO] Trevni Java Core ................................... SUCCESS [  0.345 s]
[INFO] Apache Avro Mapred API ............................. SUCCESS [  8.466 s]
[INFO] Trevni Java Avro ................................... FAILURE [  2.180 s]
[INFO] Trevni Specification ............................... SKIPPED
[INFO] Apache Avro Tools .................................. SKIPPED
[INFO] Apache Avro Protobuf Compatibility ................. SKIPPED
[INFO] Apache Avro Thrift Compatibility ................... SKIPPED
[INFO] Apache Avro Maven Archetypes ....................... SKIPPED
[INFO] Apache Avro Maven Service Archetype ................ SKIPPED
[INFO] Apache Avro gRPC ................................... SKIPPED
[INFO] Avro Integration Tests ............................. SKIPPED
[INFO] Apache Avro Codegen Test dependencies .............. SKIPPED
[INFO] Apache Avro Codegen Test ........................... SKIPPED
[INFO] Apache Avro Performance Test Suite ................. SKIPPED
[INFO] Apache Avro interop-data-test ...................... SKIPPED
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  47.480 s
[INFO] Finished at: 2024-07-24T10:18:12Z
[INFO] ------------------------------------------------------------------------
[ERROR] Failed to execute goal org.apache.maven.plugins:maven-javadoc-plugin:3.8.0:jar (module-javadocs) on project trevni-avro: Execution module-javadocs of goal org.apache.maven.plugins:maven-javadoc-plugin:3.8.0:jar failed: Unable to derive module descriptor for /home/fokko.driesprong/.m2/repository/org/apache/hadoop/thirdparty/hadoop-shaded-protobuf_3_7/1.1.1/hadoop-shaded-protobuf_3_7-1.1.1.jar: hadoop.shaded.protobuf.3.7: Invalid module name: '3' is not a Java identifier -> [Help 1]
[ERROR]
[ERROR] To see the full stack trace of the errors, re-run Maven with the -e switch.
[ERROR] Re-run Maven using the -X switch to enable full debug logging.
[ERROR]
[ERROR] For more information about the errors and possible solutions, please read the following articles:
[ERROR] [Help 1] http://cwiki.apache.org/confluence/display/MAVEN/PluginExecutionException
[ERROR]
[ERROR] After correcting the problems, you can resume the build with the command
[ERROR]   mvn <args> -rf :trevni-avro
```

pom.xml

@@ -134,6 +134,7 @@
           <version>${maven-javadoc-plugin.version}</version>
           <configuration>
             <notimestamp>true</notimestamp>
+            <legacyMode>true</legacyMode>
           </configuration>
         </plugin>
         <plugin>